There are a lot, and as difficulty have no absolute scaling, i will give you some idea of the most difficult pieces and composers.
(And among them, i will pick Liszt, Godowsky and Alkan)
Liszt (he is like the label of difficult pieces)
eg: Feux Follets, Concerto 1, Sonata in Bb.....
Rachmaninoff
eg: Rach3, Rach2, Rach Sonata, some etude tableux...
Scriabin
eg: Sonata NO.9,8,7.... etudes...
Provokiev
eg: Sonatas...etudes..
Tchkovisky(i always have problem with his name)
Eg: Tch1
Alkan(Always being ignored, but some of his work is impossible)
eg: Etude Op.39 No.8,9,10,11(Symphony for piano solo, COncerto for piano solo)... Sonata(the life sonata, with 4 movemtn about a man in his 20,30,40,50)
Godowsky
53 Chopin Godowsky Etude
44 Variations, Cadenza, and Fugue on the Opening of Schubert's 'Unfinished' Symphony (horowitz once said he avoided this piece because "he doesn't have 4 hands to play it")
Balakirev
Oriental Fantasy - Islamey(was once called the 'most difficult and impossible piece for piano solo')
Ravel
Gaspard de la Nuit: 3 Poems for Piano after Aloysius Bertrand(intended to took over the 'most difficult piano solo piece' from the Islamey by the composer, the composer couldn't even play it himself)(divided into Ondine, The Gallows, Scarbo)
Beethoven
some of his late sonata, include the most impossible one, the only piece Liszt couldn't sight-read and spent 2 months to 'study', Hammerkleiv sonata.
Henselt
His piano concerto,and etudes, Anton Rubinstein once said it is waste of time trying to master those piece. Cause Henselt was like Paganini, was a freak.
For transcription
Busoni
His bach-busoni transcription, la campanella, actually all the pagnini etudes
Horowitz
HR2,HR6,HR15,HR19, Mephisto Waltz, Rach sonata, CARMEN FANTASY, stars and stripes forever, wedding march, military march....
Cziffra
yea, the mighty cziffra, who wouldn't know the crazy OCTAVE BUMBLE BEE!!!!!! (and once again, playing the whole Grand Galop Chromatique in 2:57 is not the only inhuman achivement he did)
Hamelin
Last but not least, Busoni of our time. Made all those best and most difficult recording(include complete Godowsky etudes and Alkan's work)
his transcriptiosn are equally crazy too
Of course the famous one
Triple etude= Chopin etude Op.10 No.2 + op.25 No.4 + Op.25 No.11
Black key etudes, Rossini La Danza, HR2, La Campanella...(The craziest LC ever written)