if you`re into solo piano here are my favourites. all of these are quite difficult but i`ll give them a go:
Baroque: Bach chromatic fantasia and fugue (BEW 903)
Classical: beethoven`s appasionata sonata
Romantic: chopin polonaise in a flat major (op. 53) (the B section has really hard octaves, lots of technique there)
Modern:i like prokofiev sonata no. 3 (op. 28)
i don`t like modern much though. to much 12 note harmony, it never goes anywhere.
Also chopin etudes are wonderful.
If you need a concerto, i like the emperor by beethoven.
Anyone who can play rach3 well is guaranteed entrance. But that`s extremely difficult especially with orchestra.