Ultimate show-off pieces are not only showy and sounds harder than they are, but also good music, which a significant amount of the suggested pieces so far has been lacking. The rage over a lost penny is a HORRIBLE piece, even though it is fun. As are the Earl Wild Etudes, the Liszt Paganini etudes, the Rachmaninoff Etudes, etc.
IMHO, most of the Chopin Etudes (to repeat e60m5's comment) works very well. I'd include the "black key" (op 10 no. 5) in the list of easy etudes by Chopin (and there aren't that many), along with Op. 10 no. 8, 11, and op. 25 no. 1 and 2. Rachmaninoff preludes are sometimes good too, and they're good pieces. Op. 23 no. 5 is rather difficult but not impossible. No. 7 and 8 are wonderful (8 is a little tricky, but if you get the hang of it, it's actually rather easy, and in 7 there's a pedaling trick that makes the opening sound great). Don't play 9 unless you can do the double thirds etude of Chopin no problem.. and even then it's much harder.
One of the ultimate, and short showpieces I've played recently is the Gnomenreigen etude of Liszt (the second of a group of two, the first one's Waldesraschen). It's not that difficult - there are 2 places that are tricky, and they'll take a bit of practicing, but should be managable. If you're really up for a challenge, do Feux Follets of the Transcendental etudes. now that's fun. (I still think Rach op. 23 no. 9's harder though).
I'll post more suggestions when I think of them. Have fun!