The late, great George Carlin. I really do like his acts. The one about the little things that make us all the same is really good. He was not just funny at all of his performances, he usually proved a point too. Lewis Black is another I really enjoy.
George Carlin, hand down. Not only was the man completely hilarious but he was also brilliant; I've been laughing my, uhm, butt off and contemplating the finer points of western society at the same time when watching him.
Unfortunately I'll never get to see him live as he's no longer with us but the man was a master of the craft.
Anyway, message me if you want a link I don't want to get in trouble for posting one of his videos on the forums :P
Stephen Colbert. The Colbert Report is hilarious! It always makes me laugh. It sucks I can only watch it on the internet though. Here is one my favorite clips from the show.
Jeff Dunham and, because i feel compelled to say this, Chris Newberg because hes funny and makes fun of Piers Morgan. On an unrelated note, has anyone that watches Americas got Talent noticed the large number of British producers and judges. (Piers Morgan and Sharon Osbourn.)