... if you mean that people actually play
Rach + Prok concertos
Balakirev - Islamey
Ravel - Gaspard
Liszt Rhap. 2 (hamelin's cadenza), Etudes (some of Chopin's too)
Hammerklavier (to sound good)

thanks for introducing mazzepa, totentanz, ravel toccata
also bach-busoni chaccone (but harder for violin)

Well, in technical terms i would say Chopin Polonaise op.53 and all of Liszt's Hugarain rasphody, these can be overcome with extreme hard-efforts. I got to say, interpretation are the hardest like. Schumann pieces and Brham's ballade are hard to interpret!! That's the hardest to conquer!!!!