Speeking of prog metal i couldnt say anyone except Jordan Rudess. Has done so many inovative things in modern keyboard and prog metal..
But in prog generally Keith Emerson was a great keyboardist. open minded using one moment the craziest - spaciest effects under a psychedelic background and the next moment playing the creole dance with HIGH skills in piano... I admire him!! :-)
I'm not sure I would dare to say who's the greatest, most of these folks are playing at a very high level of proficiency in this genre.
I would put Patrick Moraz way up on my list;
Close in the running would be Herbie Hancock, who seems to be able to play in any style without losing his own. He's a marvel.
Also in there would be Kerry Minnear of Gentle Giant, who had some of the best left-right independence I've heard or seen.
Eddie Jobson is still a favorite of mine - plus the fact that he's also a violinist. That's just sick.
Perhaps not as technically gifted, Roger Powell (Todd Rundgren's Utopia) is able to emote through the synthesizer in ways I've rarely heard otherwise, and is quite the inventor/mad scientist type, to boot.
I'm trying to decide if Wakeman is a better keyboardist than he is a comedian. (That's not a put-down: He once guest DJ'd at a local radio station and was the funniest guy I'd ever heard on the air.)
Finally, no one seems to give Tony Banks enough credit. His work wasn't always showy, just complex in a "can-you-really-play-all-of-that-at-once" sort of way. And, he just sat there - the opposite of guys like Emerson, so you often didn't perceive the level of difficulty just by watching him.
Those are my guys. Speaking of which, where are the women progressive rock keyboardists?