It's been a few years since the other thread was posted in, so I figured I might as well make a new topic.
Anyways, what do you guys believe to be the hardest pieces by various composers? I'm not going to limit this to just well-known composers like Liszt. Feel free to throw in all the Alkan, Mereaux, and Xenakis you want!
Personally, (off the top of my head)
Liszt: Transcriptions of Beethoven's 9 Symphonies
Also: S. 137 Douze Grande Etudes, S. 253 Spanish Fantasy
Chopin: Op. 25 Etudes
Also: Ballade no. 4, Sonata no. 3
Beethoven: Hammerklavier
Mozart: K. 576 Sonata
Scarlatti: K. 141
Also: K. 120
R. Schumann: Toccata
Also: Symphonic Etudes, Fantasy
Brahms: Paganini Variations
Debussy: Etudes
Godowsky: 53 Studies on Chopin's Etudes
Rachmaninoff: Concerto no. 3
Alkan: Op. 39
Scriabin: Sonata no. 8
Also: Sonatas no. 5-7, 9-10, Op. 42 Etudes
Balakirev: Islamey
Ravel: Gaspard de la Nuit
Also: Miroirs, La Valse
Stravinsky: Petrushka
Mereaux: Op. 63 Etudes
Sorabji: Sonata no. 5 Opus Archimagicum
Also: Literally everything he wrote, but in particular Symphonic Variations
Xenakis: Evryali
Also: Literally everything he wrote, but in particular Synaphai
Ligeti: Etudes
Albeniz: Iberia
Finnissy: English Country Tunes
Schubert: D. 958
Faure: Ballade Op. 19
Ives: Sonata no. 2 "Concord"
Messiaen: Vingt Regards
Also: Catalogue D'oiseaux
Joplin: The Entertainer
Also: Sugar Cane Rag, Easy Winners, The Cascades
Any other works that I'm missing, or composers that I missed? (I know I didn't include Bach, but I feel that he should be left out of difficulty discussions.)