Alkan Op. 76 No. 2 is harder than any of the other etudes listed except some of the Sorabji etudes, more specifically the ones pushing 40 minutes. Etude de Concert "Le Preux" Op. 17 and "Comme le Vent" Op. 39 No. 1 are possibly literally impossible to play at tempo, not only for physical reasons of the performer but for physical reasons of a concert grand piano. The 76-2 is definitely more difficult than any movement of the Solo Concerto or the Symphonie, despite the musical pitfalls in the 39-4. It rivals the first movement of the Concerto as far as time required to play it and is only for the RH. It's also a set of variations so it gave Alkan a nice excuse to throw every nasty trick he knew into the piece (keep in mind he wrote the piece at the time where everything he was publishing was insanely difficult: the "Op. 76" can be misleading).
Of Sorabji the insanely long Passacaglia etude is probably the hardest, but I wouldn't say for sure since I don't have any experience with them.
Some of Dusapin's etudes are harder than Sorabji's. John Cage's "Etudes Australes" can be viscious also. Xenakis wrote etudes called "Seiben Klavierstucke" but then retracted them. They are much harder than anything by Sorabji also. Not to go unmentioned, Roslavets' "Studies" and Bolcom's "Twelve New Etudes" can be pretty insane, but aren't long enough to contend with Sorabji's; more difficult if time is not considered a factor. Rzewski's etudes (recording is tough to get ahold of) aren't any walk in the park either.
Ligeti etudes No. 6 and 14a are both way harder than No. 13. I'd say Desordre, Touches Bloquees, Fanfares, Automne a Varsovie, Vertige and (excuse me for the gross misspellings but I don't have the sheets sitting in front of me) Coloana Infinita and Coloana s'farsit are all harder than l'Escalier du Diable. Touchees Bloques and Fanfares are extremely tricky, but not as out-right difficult as 6/14a.
Liszt's etude "Chasse-Neige" is harder than Feux Follets. I have seen way too many awful Chasse-Neiges in competitions, but very very few "messed up" Feux Follets. Also, there have been three versions of the "Transcendental Etudes", one of them being the Douze Grandes Etudes and the other being the Grandes Etudes de Concert (or something like that). Both sets are harder to play than what you're used to. In the Douze Grandes Etudes, the Appassionata Etude (No. 10) is probably the most difficult, and would be Liszt's most difficult published etude. None of the concert etudes are particularly tough, but La Campanella and No. 6 are both quite difficult from the Paganini Etudes, but I doubt anyone would say that are as hard as the Appassionata from the Douze Grandes Etudes.
Of Chopin's etudes, 25-6 is undoubtably the most difficult to master; pianists spend years upon years trying to perfect it. 10-2, 10-4 and 25-11 are all more difficult to play at a sub-par to mediocre level though, but 25-6 is the most difficult to perfect.
Of Rachmaninov Etudes, the No. 8 "Grave" Op. 33 is much more treacherous than it sounds and would be my vote for the trickiest to play, not to mention the most commonly botched. Op. 39 Nos. 1 and 6 are the most forwardly difficult.
The four and a half minute recording of the Chopin-Godowsky Study on Etude Op. 25 No. 11 "Winter Wind" by Hamelin, a consistant speed demon, should tell you how beastly that piece is. Without a doubt the most difficult.
None of the Debussy etudes are particularly difficult except No. 7 "Pour les Degrees Chromatiques" which any pianist will tell you is the most difficult. There isn't really a contest.
Messiaen etudes kinda suck. Do Regards.
Topic: Most Technically Demanding Etude in a set