I would have to say Black Sabbath's Geezer Butler, but I also have great admiration for Pete Way (UFO), Steve Harris (Iron Maiden), Lemmy Kilmister (Motorhead) and Bootsy Collins (Parliament/Funkadelic).
Geddy Lee, bassist and lead vocalist of Rush. I've been an "on and off" fan of Rush since their inception, and the bass playing is consistent and deepley intricate. There are times I have a hard times with the tone of his voice, but his ability to play bass is tremendous. In the 60's and 70's R&B Pop stylings, Larry Graham of Sly & The Family Stone and later, Graham Central Station with his unique "plucking bass" style was also remarkable.
Duff McKagen from GnR/Velvet Revolver/Loaded is pretty awesome. But if we're talking the best of the best, I'd have to say Les Claypool from Primus. Don't much care for their music, but man, Claypool is just vicious on the bass.
I was a pro bass player for ten years, but before I set off on this career I watched and listened to Entwistle from the who, who was remarkable, and Jack Bruce whose unique style set him above and apart from the others.