Hardest Piano Piece?

[Shadows Within]

what do you think the hardest thing to play on Piano/keyboard/harpsichord is?

[Mazeppa]

Can't speak from personal experience here, but Godowsky's transcriptions of Chopin's Etudes are notoriously difficult to play. This would be for piano of course.

In fact, I might have to ask my teacher this question sometime.

[Lydisk]

some of rachmanninovs tableau etudes(sp?) are insanely hard