I'm pleased to see so few people mention Jimmy Carr and not a single mention of Jim Davidson - am I alone in finding them both completely repulsive?
As for what I like... well, it's hard to fault Dylan Moran and Bill Bailey. I'm also a big fan of Russell Brand, recent stupidity and controversy notwithstanding. George Carlin is always worth a mention. Then there's the folks from Mock The Week, they're all very funny. Charlie Brooker is great, Bill Hicks... the list is never-ending, really.
I've never seen anything of Mitch Hedberg, but he's often mentioned alongside other comedians I enjoy, so perhaps it's time for me to check him out.