I think the ones that are usually said to be hardest are _Gaspard de
la Nuit_ by Ravel (in particular, the third movement) and Balakirev's
_Islamey_.  I imagine Sorabji's _Opus Clavicembalisticum_ must be very
demanding, if only of the player's stamina.

Virginia Knight
[log in to unmask]
Personal homepage: http://www.ilrt.bris.ac.uk/~ggvhk/virginia.html