In terms of overall skill and influence on drumming as an instrument, hands down its going to be Neil Peart and John Bonham. But different genres require different types of skills and practice, so its going to be difficult to say overall who the greatest drummers are.
But in terms of just flat-out ability and influence, it's gotta be Bonham and Peart. Though I'm also a big fan of Bill Berry's drumming, and how it influenced the alternative rock genre.
Keith Moon treated drumming as if he was getting high off of it. He would just go faster and play louder. Amazing to say the least.
Ringo's up there as well. I mean, if you listen to his playing on Rain and A Day In the Life, you hear some great fills and snare rolls. Great. He also played to some funky time signatures with The Beatles.
Both are also exceptionally underrated. Maybe had Moon lived longer, he'd get more credit, but Ringo has a bad rap, a lot of people seem to think he's a terrible drummer for some reason. Mostly because if you hear it from someone else, it must be true!