No one mentioned Chopin scherzo #2, Rachmaninoff etudes, or Liszt's Mephisto Waltz, which I think is much easier than it sounds.
Some of these replies are embarassing! I thought this site was for pianists! Judging by these posts there are alot of seriously bad pianists who are offering NO HELP WHATSOEVER (with some exceptions)!!! Examples such as gaspard de la nuit and islamay are ridiculous- i've been playing these since my mid-teens and I still fear them. These are NOT show-off pieces- these are genuinely hard pieces if not the hardest which even the best pianists struggle with.
In any case a show-off piece sounds awfull if it's played in a laborious manner. Will.
The Rachmaninov Etudes are not horrible pieces,Ed (horrified)