I'm gonna go with Carlin on this one. He's the kind of guy that always makes me laugh somehow, no matter what he says. He was very intelligent, and intelligent humor is the best kind if you ask me.
After that, I'm gonna go with Bill Hicks. The "calm and well-paced asshole" routine was brilliant, and he truly meant every word he said while also making a point.
On the negative side:
While I can't say anything bad about the guy, he's got a really obnoxious fanbase. They make him come across like god's holy gift to the universe, and like the guy invented every single joke out there and everyone ripped him off after that. And if you speak against that, you are automatically wrong and probably likes Dane Cook.
While I can agree with this, I think they're making too big a deal of it. Yes, I know Denis Leary stole quite a bit of his jokes for the No Cure for Cancer special. I still laughed my ass off at it. The Hicks fans still make him come off as the devil incarnate for that.
But whatever. Hicks and Carlin were awesome, and it's a shame they're gone. Then again, I wonder what they'd think if they saw today's society.