poulenc toccata. i haven't started it, but it looks like a challenge. it is in three (trois pieces) prelude, hymn, and toccata.
barber's fourth nocturne isn't easy either, and it's greatly improving my sightreading (still finding naturals or flats that are carried over in the measure that i forget - esp if one is in the right hand and another in the left). just today, i figured out a better fingering than is in the book from the previous pianist. i grab a few notes with the rh thumb (makes it much easier and smoother, i think). anyway, someday, i will play it.