Wow, this thread is still going? Lot's of great names here!
Okay, I'm in.
I'll second (third?) the votes for Ringo (very underrated, listen to the way he holds back the beat in "Ticket to Ride"... Brilliant.
Neil Peart is a master - after being way out there, he somehow always finds a way to get back (an right on!) the downbeat.
Keith Moon was fabulous.
I guess my personal favorite is (surprise, surprise) John Bonham (groover extraordinaire), but a close second is a guy you have to listen to (go back and do it again, though I'm sure you've heard the songs, but even if you haven't already) - BJ Wilson of Procol Harum. The greatest tom fills, he knew not only what and when to play, but what and when NOT to play. A sparse, yet powerful style. Have a listen. Sadly, he passed away several years back.