A jester in the emperor-has-no-clothes sense. Best routines questioned why we use language the way we do. Got better as he got older. Cons: Some will only know him for the seven words (long since updated) you can't say on television.
Not just one of the smartest comedians ever, but one of the smartest human beings ever. His humor was a dope-slap to everything half-assed in our world. He didn't break taboos, he annihilated them. Cons: He could offend just about anyone, but especially those who are socially conservative or sensitive to "gross" humor.
A true comedic genius, social philosopher, and somewhat of a prophet. Ahead of his time and greatly appreciated. For the Carlin newbie, I recommend his "Wtf am I doing in New Jersey?" show. Cons: Rest in peace, you old f***. We will probably never see the likes of you again.