I would contend that Steve Martin is the greatest comedian of all time. He revolutionized the industry for later generations. When other comedians were talking about the world and making observations, Steve was up there pickin on his banjo singing songs about his grandmother. The guy just got up onto that stage and acted a fool. And that is what was so brilliant about his routine. He just acted silly all of the time. And he was incredibly talented in other areas as well. He is a talented musician, writer, actor, comedian, and all around great guy. Carlin can't even touch Steve Martin when it comes to being a Renaissance Man. Not only does his (early) filmography speak for itself, but he was also one of the major pioneers of SNL back in its glory days. He has mellowed out recently, mainly spending his time playing in blue grass bands, but it wasn't so long ago that everything he touched turned to comedic gold.
Honorable mentions would go to Eddie Izzard and Louis C.K.