Worst comedian? Some of my favorites have been named here as worst. No point in arguing…de gustibus, etc.
Of those I've seen, I'd vote for Tom Green. He played some really nasty pranks on people in public and videotaped them. For instance, going out in public in a wheelchair, falling out of the wheelchair, tricking some unsuspecting bystander into trying to help him get back into his wheelchair, falling out again, etc. Real disgusting.
Jay Leno has also offended me with tasteless jokes about JonBenét Ramsey and her family. When he's at his best, he's pretty dull. I guess I resent him more because so many other people like him.
It's not that I'm easily offended. Quite the contrary. I like people who push the envelope of what's socially acceptable—but they've got to be funny, and they can't pick on people unfairly.