From the purely technical viewpoint the most difficult music to play might be the one not designed to be played by human. Conlon Nacarrow music written for pianola was impossible to play because of speed and impossible fingering. I've heard that today few pianists can play it:
http://www.youtube.com/watch?v=LFz2lCEkjFk
http://www.youtube.com/watch?v=LFz2lCEkjFk