Robin Williams. His comedic genius is unparalleled, IMO. The first time I watched Robin Williams Live on Broadway, my sides constantly hurt from laughing so hard.
Guys like George Carlin are great, but too often the humor is lessened when I feel they're wrong. You can make a political joke that's hilarious even if inaccurate, but too often it becomes clear that they don't really know what they're talking about, but they think they do.
If someone makes a joke, and it's basically "I don't understand, but this seems really weird to me", it can be hilarious. But if they do the same thing, but it comes across as "Here are these people who believe this really weird thing, and let's laugh at them", it can be kind of awkward, especially if they don't believe it.