ok, the problem with this thread is that a lot of the greatest pieces ARE the hardest.
This is because to take full advantage of the piano and it's power, you often need bombastic parts.
The beauty of the piano is that it can orchestrate, and even the most powerful music can be played by one person.
I don't care how hard it is, Gaspard de la Nuit is probably my favorite piece just for its sheer beauty and power (and genius in every way). The Liszt sonata is also genius... and powerful, it's not just a show piece like some of the Hungarian Rhapsodies.
Anyway, my favorites (difficulty aside) would be
Barber - Sonata
Ravel - Gaspard de la Nuit
Beethoven - Opus 31 no 2 (Tempest), Op 109, Op 106 (Hammer K), Opus 57 (appassionata)
Chopin - Sonata no 2 and 3, Polonaise op 44, Grand Polonaise, Trioseme Ballade
Liszt - Sonata, Valee de Obermann, Dante Sonata
Saint-Saens - Thirds etude
Rzewski - The People United Will Never Be Defeated
Scott Joplin - Palm Leaf Rag
Rachmaninoff - .... too much to name for me. 1st and 2nd Sonatas are epic... Rachmaninoff's music is the most potent to me.
Bach - Partita no 2 in C minor
Schumann - Fantasie in C, Sonata in F sharp minor