I'd have to say the late George Carlin, but a close second is Christopher Titus. If you haven't heard of him, look him up. This man is the guru emotional trauma and making it hilarious.
Behind those start the procession of the greats who fall just short of legend in my mind: Lewis Black, Mitch Hedburg, Katt Williams, and others mentioned on the above posts.
Seriously though, Titus is one of those guys who'll make you either laugh till you cry, or just cry. If you've gone through some serious shit in your life, you know that "normal people freak out when the shit hits the fan; screwed up people, we step to the side of the fan."
[edit] Youtube "Norman Rockwell is bleeding"