There is one classic that is more coveted among the climbers than any other and in the dreams of any Ardennes expert. Sunday is the most important day of the spring for many of the world's most formidable bike riders as the classics season comes to a close with the oldest of the one-day races: the Liege-Bastogne-Liege.

It is no coincidence that almost all of the world's best climbers and stage race experts are travelling to the Belgian city of Liege these days to mix it up with the one-day specialists who have battled it out on the slopes of the Ardennes in the Amstel Gold Race and the Fleche Wallonne. While the first two of the hilly spring classics are suited to riders of an explosive nature, the last of the three races in the Netherlands and Belgium is much more of a climbers' race.

What characterizes most stage race experts are their strength on the climbs and a formidable endurance and that is exactly what is needed to come out triumphant in Sunday's Liege-Bastogne-Liege. For once the key attributes in a classic are not an ability to handle the constant battle for position or to sprint up short, steep climbs. On Sunday you need the ability to keep going and wear down your opponents on a long, hard day with numerous climbs which are longer than the ones found in the other Ardennes classics.

These factors give the Liege-Bastogne-Liege its unique position on the cycling calendar. It is the one opportunity of the year to see such a strong line-up of classics specialists and grand tour riders going head to head on almost equal terms. The Tour of Lombardy shares some of the same characteristics but the Italian classic is held at a time where many riders have ended their season and the start list is not nearly as impressive as the one found in Sunday's race.

One reason for the esteem of the Belgian race is its nature and characteristics. The other major factor is the deep history of the race. First held in 1892, "La Doyenne"  is the oldest among the classics and has an impressive list of winners to boost its significance. It almost seems to be a God-given fact that the greatest of all riders, Eddy Merckx, tops the rankings with no less than 5 triumphs in Liege.

The race is the fourth of cycling's 5 monuments, the most important one-day races of the sport, and it is without any doubt one of the most prestigious in this very exclusive group. It also plays a crucial role for the most important sport in one of the most split countries in the world. While the Flemish people see the Ronde van Vlaanderen as the highlight of the classics season, their Wallonian compatriots see nothing coming even close to the status of their Ardennes battle.

As the hardest of the one-day classics, the race fits perfectly into the overall structure of the cycling season and the anatomy of the Ardennes classics. The hilly one-day races have seen a gradual changing of the guard as more and more climbers have arrived in the classics heartland while more and more cobbles specialists have taken a well-deserved rest. While the strong men of the Tour of Flanders and the Paris-Roubaix could still play a role on the narrow Dutch roads and short climbs in the Amstel Gold Race, they have nothing to do in Liege on Sunday. And as the hard climbs make a much more natural selection and gradual elimination in La Doyenne, positioning is not as important as it has been in the previous races and so the race favourites are much more in need for climbing domestiques than strong riders. Hence, it will be a start list consisting almost only of riders who specialize in uphill sections.

Furthermore, the race marks a perfect transition from the classics season to the time of the grand tours which kicks off in a few weeks time with the Giro d'Italia. With the stage race experts and classics specialists all gathered on the same start line, it is another perfect example of the beautiful anatomy of the cycling calendar.

Last year the race delivered the biggest surprise of all the classics as Maxim Iglinskiy (Astana) made a fantastic comeback and caught a fading Vincenzo Nibali (Liquigas) who seemed to have almost locked up his first classics victory. With Enrico Gasparotto sprinting home to take the final podium spot after his victory in the Amstel Gold Race one week earlier, the race ended a fantastic and highly unexpected Ardennes campaign from the Astana team. This year Iglinskiy is back to defend his title but this time he will line up in the same team as the man most eager for revenge as Nibali arrives from Trentino to take the victory that eluded him last year.

The course

The 261,5km race is one of the few classics - Milan-Sanremo is another - which really keeps its tradition of travelling between the cities that make up its name. The race starts in the industrial city of Liege and travels south into the heartland of the Belgian Ardennes.

As it is the case in most of the classics, the opening part of the race is not overly difficult and while it is impossible to traverse the region without going up and down most of the time, the 98km from Liege to Bastogne only take in one categorized climb, the Cote de la Roche-en-Ardenne (2,8km, 6,2%) after 70km of racing. This part of the race mostly serves to accumulate fatigue and to see the creation of the day's early break.

When the riders reach Bastogne, they turn around to head back to Liege but this time they take in a much longer and harder route. Having climbed the Cote de Saint-Roch (1,0km, 11%) after 116,5km, the racing gets serious  when they hit the Cote de Wanne (2,7km, 7,3%) with 101,5km to go. From then on the 9 remaining climbs come in quick succession and there will be no time to recover during the remaining part of the course.

This is the time for the strong teams to start to apply the pressure, and from then on it is a gradual elimination race as every climb sees plenty of riders drop off under the hard pace. Meanwhile, the race is usually very aggressive in this phase. It is not uncommon to see the creation of a break from which the strongest are able to remain in contention deep into the final.

The race reaches its landmark climb the Cote de La Redoute (2,0km, 8,8%) with 38,5km still to go. While the steep slopes were often the scene of the major selection in the earlier editions of the race, the much more controlled nature of modern-day racing has meant that the climb is now located too early in the race for the favourites to really hit out. Nonetheless, the severity of the ascent still creates a huge selection and at the top of the Redoute, the number of contenders has been drastically reduced. Meanwhile, the climb remains a perfect opportunity for riders just below the front row to hit out in an attempt to try to surprise the favourites.

With Redoute playing a less crucial role, organizers ASO chose to introduce a new climb in 2009. The steep Cote de la Roche-aux-Faucons was located inside the final 20km of the race and since then it has been the place for the favourites to drop their competitors. In 2009 Andy Schleck soloed off the front to take an emphatic victory, in 2011 the decisive move with the Schleck brothers and eventual winner Philippe Gilbert went clear on this climb and last year Nibali put down the hammer and dropped all of his companions on this crucial ascent. Only in 2010 when Alexandre Vinokourov won, the climb failed to create a major selection.

This year road construction has forced organizers to remove the climb from the route and it will be replaced by the Cote de Colonster (2,4km, 6%) whose summit is located with 17km still to go. While the new ascent is longer, it is not nearly as steep as the Roche-aux-Faucons and it could very well turn out to be much more difficult to create a selection on this new hill. This may create an easier edition of the race and could fall into the hands of the riders with a strong sprint.

As usual, the riders have one final chance to drop any companions as the Cote de Saint-Nicolas (1,2km, 8,6%) is once again located with just 5,5km to go. This is usually the scene of the final attacks of the race, and the steep slopes can create plenty of damage late in a long, hard race.

After the top it is a short descent until the final 1,5km to the finish line in the Liege suburb of Ans. This final section has a gradual incline and while it is not one of the categorized climbs of the race, a 4,6% average gradient in the final kilometer makes for a hard finish to an already very hard race. It is not always the fastest who wins a sprint into Ans but more often the one with most power left in his legs.

The weather

Riders will be happy to see that the spring-like conditions which have characterized the Ardennes classics after an unusually cold early part of the season, will once again greet the riders on Sunday. This will be in sharp contrast to last year's race whose outcome was dramatically influenced by rain and hail just before the riders had to climb La Redoute.

It will not be as warm as it has been in Amstel and Fleche but 12 degrees Celcius and plenty of sunshine will still be pleasant for a peloton which has mostly raced in freezing cold this season. Furthermore, there will be almost no wind which should make for a less stressful race.

However, the wind will come from a northerly direction which means that riders will enjoy a tailwind during the opening run to Bastogne while a headwind will greet them when they turn around to tackle the final 163,5km back to Liege. Even though the wind is not strong, this may be another factor to work against the intentions of the race's attackers.

The favourites

The Liege-Bastogne-Liege has historically been one of the least predictable classics and this year seems to no exception. While most of the year's biggest one-day races have had major favourites, La Doyenne seems to have multiple potential winners.

One reason is the impressive depth of the field with its fusion of classics stars and stage race experts bringing two different worlds together on a course that suits both, another is the final part of the race which is much less controllable by often isolated leaders and thus much more favourable to attacks than most other classics. Finally, nobody can underestimate the unpredictable role of fatigue at the end of a long and hard spring season.

Nonetheless, one rider marks himself out as the number one favourite. Alejandro Valverde (Movistar) has been on fire all season and no classic is more suited to the Spaniard than the Liege-Bastogne-Liege. With two victories in La Doyenne, no active rider has as many wins in the race as the Movistar captain and this is a further testament to his ability on the steep slopes south of Liege.

Last year he returned to his preferred stomping group after having missed the race in 2011 due to his suspension. He paid a costly price for the long lay-off and with severely reduced endurance, he was nowhere to be seen at the end of the longest races. A year of competitive action has brought him back to his best and his 3rd place at last year's worlds and 2nd in Sunday's Amstel Gold Race proves that he is once again competitive on longer distances.

This year he has even timed his form to perfection after having been incredibly strong in February and March. Illness forced him to miss his final preparation race for the Ardennes in the Klasika Primavera and he had some doubts when he arrived in the Netherlands ahead of last Sunday's first classic. He was reassured with a strong 2nd place in which he was able to close down the hard acceleration from Philippe Gilbert (BMC) on the Cauberg and he used his usual wily tactics to benefit maximally from the situation. He disappointed somewhat in the Fleche Wallonne but the Spaniard has never been a man for the hardest battle for positions and he entered the final climb of the Mur de Huy some way back in the peloton. Nonetheless, he climbed back to end up 7th on the day.

The course suits him down to the ground. As a real climber, the longer ascents are much better for him than the ones found in Amstel Gold Race and he has the clear advantage of being one of the fastest finishers on the final rise into Ans. While other "sprinters" like Philippe Gilbert and Simon Gerrans may be worn down due to the tough course, Valverde should only get stronger as the race goes on and hence he should be able to outkick his rivals in a final dash to the line. Furthermore, he will be advantaged by the revised course as the missing Rouche-aux-Faucons climb removes the most obvious scene for attacks from the most in-form climbers and it could be hard to drop Valverde on the Colonster ascent. Furthermore, he is assisted by a really strong team with Rui Costa, Nairo Quintana, Andrey Amador and Giovanni Visconti all able to be present in the lead group deep into the final. The question is whether they will be able to control the race and deliver their captain at the bottom of the Saint-Nicolas climb in a perfect position. If this is the case, the Spaniard will be very hard to beat in the final 5,5km of the race.

No one can doubt the current level of Vincenzo Nibali (Astana). He arrives in Belgium fresh from a dominant stage victory atop the steep Sega di Ala climb in the Giro del Trentino and no one is more eager for revenge than the Italian. Last year he was by far the strongest on the climbs in the oldest classic and nobody was able to follow him when he put down the hammer on the Roche-aux-Faucons. However, he faded towards the end - just as he had done it 6 months earlier in the Tour of Lombardy - and was passed by a strong Maxim Iglinskiy just before the line. This year he has marked out La Doyenne as an important objective as he approaches top condition ahead of his main target, the Giro d'Italia.

However, Nibali has one crucial weakness which will continue to haunt him in the one-day races throughout his career. Despite his immense strength on the climbs, it is very hard for him to win as his (lack of a) sprint is useless against his much faster competitors. The slight rise in the final kilometer is of course to his advantage but his only real chance to win the race is to arrive in solo fashion. The race has often been won by a lone escapee but this year Nibali will dearly miss the Roche-aux-Faucons and despite his strength it is highly uncertain that he will be able to drop his rivals on the less steep slopes of the Colonster climb. He will probably have to wait to the Saint-Nicolas while using his impressively strong team with the likes of Iglinskiy, Enrico Gasparotto and Jakob Fuglsang to toughen up the race early on. However, his recent strength means that he will be a marked man and he will not be allowed any leeway from his main rivals. He simply has to prove superior power if he is to win the race but no one can rule out the possibility that this will be exactly what happens.

Sergio Henao (Sky) has probably been the strongest on the longer climbs during the month of April. He seemed to be the best in the Basque mountains in the beginning of the month and he has been right in the mix in both the Amstel Gold Race - which does not suit the tiny Colombian - and on the Mur de Huy last Wednesday where he was only beaten by a surprisingly strong Daniel Moreno (Katusha). The Sky rider will undoubtedly relish the longer climbs in Liege and his 5th place in last year's Tour of Lombardy, 9th in the world championships road race and 6th in the Amstel Gold Race prove that the distance will be no problem for the Colombian.

He may not have the same kick as the likes of Valverde, Philippe Gilbert or Simon Gerrans but he certainly has some punch in his legs and should be able to beat many of his competitors in a final dash to the line. Nonetheless, he still has to drop his fastest rivals earlier in the race and like Nibali he will miss the Roche-aux-Faucons climb. However, he could very well be the man to follow an acceleration by Nibali on the Saint-Nicolas climb and if this happens, nobody can exclude the possibility for a breakthrough victory from the 25-year-old Colombian.

As usual Sky line up one of the strongest teams and the race's big dark horse is in the ranks of the British team. Chris Froome makes his return to competitive action after a long break initiated after his win in the Criterium International and nobody really knows just how strong the Brit is right now. However, last year's Tour runner-up has made it a habit to be competitive whenever he races and while his final spring target is likely next week's Tour de Romandie, he has not travelled to Belgium for no reason.

He has been on fire all year and won the overall in both the Tour of Oman and the Criterium International, and only a bad day on the epic, penultimate stage in the Tirreno-Adriatico took away what seemed to be a dominant victory after his display on power on the Prato di Tivo mountain. He is not the perfect classics rider but in a race where climbing strength and endurance play a crucial role, it will be a huge mistake to discount the Brit. Like Nibali and Henao he will have to escape somewhere along the course and he will be another to be handicapped by the absence of the Roche-aux-Faucons. However, at his best he may the man to go off solo on the Saint-Nicolas and as his victory in the Planche-des-Belles-Filles stage of last year's Tour de France proves, he has a certain kick in a final sprint. There is no doubt that the Sky duo of Froome and Henao want a hard race and with a line-up consisting of the likes of Vasil Kiryienka, David Lopez, Richie Porte, Kanstantsin Siutsou, Jonathan Tiernan-Locke and Rigoberto Uran, they certainly have the team to wear out the opposition throughout a long day of racing.

Philippe Gilbert (BMC) is another danger man in tomorrow's race. La Doyenne is his dream race as it takes place on his home roads in Wallonia but unfortunately it is also less suited to him than the Amstel Gold Race and maybe even the Fleche Wallonne. Even though the world champion handles ascents fairly well, he is not a pure climber, and the Liege-Bastogne-Liege is a race for climbers. Nonetheless, he has proved in the past that he has what it takes to come out victorious in Ans as he won during his amazing 2011 spring campaign but even that year he was not able to get rid of the Schleck brothers and only won due to his superior sprint.

This year he seems to be much stronger than last year but he remains a shadow of the superior force that crushed the opposition in all of the hilly spring races in 2011. Last year he lost out in Liege due to a lack of endurance but as he was certainly the strongest in the final part of the Amstel Gold Race, he seems to have overcome this difficulty. However, he will not be able to just sprint from bottom to top of the longer climbs in Sunday's race and the world champion will have to take a defensive approach in which he will hope to remain in contention at the bottom of the final rise to the finish line into Ans. If he is still present at that point in the race, the nature of the finishing straight makes him a danger man.

The man most likely to challenge Valverde in a final dash to the line is Simon Gerrans (Orica-GreenEdge). While the Australian mistimed his condition last year, his preparations for this season's Ardennes classics have been close to perfect. His stage victories in the Volta a Catalunya and the Vuelta al Pais Vasco marked him out as an obvious contender and even gave indications of an improved sprint for the already fast Australian. As the Ardennes classics finally kicked off last Sunday, he confirmed his potential by being almost able to follow the furious acceleration on the Cauberg climb and he seemed to be the best of the rest behind Gilbert and Valverde.

Unlike most of his competitors he chose to skip the Fleche Wallonne as the steep Mur de Huy is too much for the 2012 Milan-Sanremo winner. He faces the same difficulties as Gilbert as he is no pure climber and there is no doubt that his explosive nature is much more suited to the Dutch classic. Nonetheless, he was 6th in the 2009 edition of the race and has been at the pointy end of the race ever since. He will be another to take a cautious, defensive approach to the long climbs and he will relish the absence of the Roche-aux-Faucons. Like Gilbert he hopes to be able to follow the pure climbers in the final of the race and if he is still in contention on the final rise into Ans, he could very well take home his second victory in one of cycling's monuments after last year's Milan-Sanremo triumph.

Joaquin Rodriguez (Katusha) is another rider surrounded by plenty of questions. Nobody really knows just how strong the Spaniard is as his crash in the Amstel Gold Race forced him out of last Sunday's race and severely hampered his chances in his favourite classic, the Fleche Wallonne. Despite his injury, he managed to finish 6th on the Mur de Huy behind his victorious teammate Daniel Moreno and this proves that his condition is certainly not bad. On paper he is well-suited to the Liege-Bastogne-Liege - he won the Tour of Lombardy last year and the Italian classic is the one most similar to Sunday's race - but for some reason he has been a major disappointment in both 2010, 2011 and 2012 and his last top result was his 2nd place in the 2009 edition when he was only beaten by a strong Andy Schleck.

If he is at his best, he will be almost impossible to drop on the climbs and he has the explosiveness to be competitive on the final rise into Ans. However, the finishing straight is not steep enough for him to be able to beat a rider like Valverde in a sprint and so he will have to go on the attack somewhere along the course. Unlike Nibali, he is not forced to arrive at the finish line in solo fashion and if he could find some allies along the way - and if he has completely recovered from his injuries - tomorrow could be the day to take home his third big classics win.

Finally, Daniel Martin (Garmin-Sharp) deserves a mention. With explosiveness, endurance and a fast finish the Irishman has all the characteristics to be competitive in the Ardennes classics but allergy problems forced him out of contention in the spring for a number of years. However, a second place in the Tour of Lombardy, victory in the Tre Valli Varesine, a 2nd in the Giro dell'Emilia and an overall Tour of Poland victory taken home on an Ardennes-like climb clearly underlined his potential and last year he had finally solved his health difficulties. With a 6th place in the Fleche Wallonne and a 5th in Liege, he was immediately a key protagonist in the Ardennes classics and this year he seems to be even stronger.

He won the Volta a Catalunya in March in impressive fashion and he bounced back from a late puncture to be just centimetres away from a podium spot on the steep slopes in the Fleche Wallonne. Had it not been for a crash, he would without any doubt also have been in the mix in the Amstel Gold Race. The longer climbs in Liege suit a climber like Martin perfectly and his fast finish makes him dangerous in a sprint from a late escape. Somewhat underrated by his competitors, the Irishman could be the man to exploit any hesitation from the favourites in the final part of the race to go clear and take the biggest victory of his career.

***** Alejandro Valverde

**** Vincenzo Nibali, Sergio Henao

*** Chris Froome, Philippe Gilbert, Simon Gerrans, Joaquin Rodriguez, Daniel Martin

** Michal Kwiatkowski, Bauke Mollema, Jelle Vanendert, Ryder Hesjedal, Daniel Moreno, Alberto Contador, Samuel Sanchez, Carlos Betancur, Björn Leukemans

* Roman Kreuziger, Greg Van Avermaet, Enrico Gasparotto, Pierre Rolland, Giampaolo Caruso, Michele Scarponi, Igor Anton, Rinaldo Nocentini, John Gadret

The climbs

Km 70.0 - Côte de La Roche-en-Ardenne - 2.8km, 6.2 %

Km 116.5 - Côte de Saint-Roch - 1.0km, 11 %

Km 160.0 - Côte de Wanne - 2.7km, 7.3 %

Km 166.5 - Côte de Stockeu (Stèle Eddy Merckx) - 1.0km, 12.2 %

Km 172.5 - Côte de la Haute-Levée - 3.6km, 5.7 %

Km 185.0 - Col du Rosier - 4.4km, 5.9 %

Km 197.5 - Côte du Maquisard - 2.5km, 5 %

Km 208.0 - Mont-Theux - 2.7km, 5.9 %

Km 223.0 - Côte de La Redoute - 2.0km, 8.8 %

Km 244.5 - Côte de Colonster - 2.4km, 6 %

Km 256.0 - Côte de Saint-Nicolas - 1.2km, 8.6 %