I seem to recall Aurélie Dupont of the POB saying that for her Nureyev’s Raymonda is the most technically demanding. And the danseurs often talk of all the punishing male variations Nureyev added to the classics such as the long solo for the Prince in his Sleeping Beauty!
Ashton’s Sylvia is also very demanding on the ballerina. There is hardly any padding and she dances almost non-stop in the ballet. I also feel that Ashton’s choreography is very exposed- the tiniest mistake is very noticeable to the audience. His choreography is also so intensely musical that if she misses a beat it’s very obvious. Monica Mason described the solos in Act I- with all the jumping, turning and balancing- as being a marathon for the ballerina! And of course no matters how tired she is by Act III, she still has to execute that pizzicato variation and be the definition of radiance during the pas de deux. And this is not to mention the interpretative side- she has to portray all three facets of the character convincingly and make all the transitions between the Acts believable!
For the men, I suppose it depends. Jonathan Cope, for example, said that a ballet like McMillan’s Mayerling is like a marathon whereas something like a classics such as Swan Lake is like a sprint. In Mayerling, Crown Prince Rudolf has to partner at least six different women (Princess Louise, Princess Stephanie, Marie Larisch, his mother Elisabeth, Mary Vetsera and Mitzi Casper) and you have to give it your very all emotionally in the last Act. But Cope said, with the classics, one has to be perfection in all the variations which can be extremely taxing even if the solos only last a few minutes.
For the corps, the Kingdom of Shades must be it for the reasons you all described! For the RB version, I believe the lead girl performs 39 arabesques (!) Nijinska’s Les Noces is also meant to be very demanding and unforgiving. The corps dancers often say the music is extremely difficult to count.