Thought I'd throw in some of my favorites (most of which I can play, but that's not the point):
- Bach's fugues and preludes that fall under the Well Tempered Clavier
- All of Beethoven's Piano Sonatas, but especially #8 (Pathetique), #14 (Moonlight) and #23 (Appassionata)
- All of Mozart's Piano Sonatas. K545 is fun to play because there is a lot of opportunity for variation in tempo and melody.
- All of Chopin's Nocturnes and Polonaises. My favorite nocturnes would be Opus 9 Number 1 and Opus 26 Number 1.
- All of Hummel's Piano Sonatas.
Ahh hell... there are too many. I'd also have to put piano concerti into this list, and my favorites are from Mozart (especially 7, 10, 17, 19, 20, 21, and 23), Beethoven (all of them) and Hummel (all of them). I lost interest in the piano concerto once you get into the romantic period; Rach has some ominous-sounding piano concerti (namely the 3rd of course), but to me they lack the emotional depth and quiet dignity of Mozart and Hummel. Beethoven's piano concerti were special because he covered such a wide stylistic period in only 5 pieces. His first is almost entirely Mozartian in style, and his 5th entirely Beethoven; in between we have a lot of interesting transitional work.