George Carlin is my favourite comedian of all time, and he always will be. I discovered him at a young age and without him, I don't think I'd be the same person I am today, or at least not as far in my development.
Bill Hicks, much for the same reason as Carlin. I discovered him much later, only a little over two years ago. I wish he'd stayed alive longer than he did.
Dave Chappelle. I don't consider him a thinker quite in the same league as Carlin and Hicks, but definitely up there. His observations are keen, and really, his delivery itself deserves a mention. Every comedian here has perfect delivery for what they do, but his is... there's something about it.
Lewis Black, much for the same reason as Hicks and Carlin. I haven't heard a whole lot from him, but I loved what I did.
Demetri Martin. I just think he's funny, kind of airy. When I listen to Carlin and Hicks and Black in a bad mood, they leave me in a better mood because by showing me that no situation is so terrible that you can't make light of it ("Fuck you, I think rape is hilarious, how do you like that?"). Demetri makes me feel better just for his sort of lofty onstage persona and almost offhanded observations that you only realize after must have been so finely tuned in order to feel so natural and calming. It's almost like watching perfectly animated water. You know it must've been hard as hell to produce, but goddamn is it a sight to behold.
Steven Wright. As far as I know, he's not an intellectual heavyweight, but goddamn he's funny, and his deadpan voice makes it all the better.