What is the hardest piano piece to play?

Question:

What is the hardest piano piece to play?

And why?

Best Answer - Chosen by Asker: According to some pianists, it's "Scarbo," the final movement from _Gaspard de la Nuit_ by Maurice Ravel. Scarbo is the hardest of the three movements, and the entire composition was intended to be harder to play than _Islamey_, a famously difficult piece by Balakirev which even Liszt couldn't manage.

You can hear a recording of Scarbo at http://server3.pianosociety.com/protecte...