I've seen a number of my faves mentioned but here is a list that comes to mind for me:
Jon Lord- The backbone of Deep Purple and the finest Hammond organ player ever.
Elton John- The man who reintroduced the piano to rock and roll. Elton is quite possibly the most famous piano player in popular music.
Amanda Palmer- When you can pair the keyboard with aggressive, expressive drumming and somehow produce some of the best music in the last decade that is an accomplishment. Despite how bad a duo consisting of a drummer and piano player may seem on paper Amanda Palmer uses her primary instrument perfectly in her own style.
Liberace- In a time when The Beatles, Elvis, and Zeppelin were all on top of the world Liberace was the highest paid performer in the world and he did so without singing, without a backing band, and for the most part even playing original songs. His success was solely from his piano playing.
Beethoven- DUH
And my personal favorite...
Yoshiki- the drummer, songwriting, and OF COURSE pianist for X Japan is in my opinion the greatest people to ever diddle the ivory. His compositions are grand and beautiful, his style of playing is so emotional and expressive, and he is finest example of classical music being perfectly integrated into the rock medium.