This is a tough one, since keyboards encompass a broad range of expertise. As far as rock I'd break it down by instrument.
Synth-Bernie Worrell (honorable mention on clav)
The all around guy I'd choose..and it's hard, but I'd say Roger Powell. He was freak level, and recorded all-time tracks on several of the above, Emerson right there as well.
If we're including jazzers then Chick Corea by a furlong with Herbie at #2.
This post was edited on 3/5 at 7:10 pm