That's a big statement, I'm a prog. drummer myself, I know and love a lot of them. Nick D'Virgilio (Spock's Beard) is one of my favorites, another one is Neil Peart (Rush), then you have Mike Portnoy offcourse, and since he does Porcupine Tree, Gavin Harrison who has done 'progressive' drumming within the progressive genre.
I think Neil Peart is one of the best drummers of all time, regardless of which type of music, but there are so many. For what I've heard the KC guy, Bill Bruford, is great, but as great as Peart, I'd doubt it.
Things are subjective as hell offcourse.
Ever listened to ELP's "Knife Edge"? If there is one drummer I really appreciate it is the drummer from Kingston Wall.