Piano players: Which is the most difficult piece you have mastered?

Question:

Piano players: Which is the most difficult piece you have mastered?

Classical or contemporary... piano is a difficult instrument, and there are pieces of music which are much more difficult than others to master.

Best Answer - Chosen by Asker: Either Chopin's Fantaisie-Impromptu or Mendelsohnn's Rondo Capriccioso. I played the Mendelsohnn more recently. I took a two-year break from piano and am just now coming back to it, with a completely new perspective.