Here's a site that has the five hardest pieces and a video of one that is supposed to be THE hardest.
Five Most Difficult Piano Pieces (Classical) | Pianist Corner ...
What do you think the hardest piano classical piece?
My vote is for the cadenza ossia from Rachmaninov's Piano Concerto No. 3. It's so difficult that Rachmaninov composed a simpler version of it that many pianists perform today. But the original sounds better:
Beethoven
