Many will say Carrot Top but I must confess that I have not seen any of his stuff. Life is much too short.
That being said, I'd have to say Andrew "Dice" Clay. Generally, as a rule, it usually does not work when you are setting out specifically to offend and shock people and Clay does not do anything to change that. He could have easily used his stand-up persona as a way of critiquing male machismo and egotism but didn't really have the balls (so to speak) to do such a thing. This is why so much of his comedy is just so uncomfortable to watch. He has "shocking subjects" and doesn't do anything to make them funny. He's just pointing various aspects of the male machismo psyche and expects that alone to be funny.
Mysogyny is definitely a problem in society and is fair game for a good satirical drubbing, but "Dice" Clay didn't do any of that. He was just yelling out shocking things in an offensive manner and hoped that that alone would be funny. It wasn't.
Anyone who actually makes people laugh can't be a "worst" standup. Lots of people like him - it may not be your brand of humor, but you can't deny that people like him. I've been to comedy shows where the comedians absolutely BOMBED - no one laughed -- maybe the occassional chuckle, but it is SO uncomfortable sitting through that. That alone qualifies as worse than Andrew Dice Clay.
I went to my cousin's comedy show a few months ago and the guy before her was awful. He joked about pedophilia and cancer (with my mother who was completely bald from chemo sitting right in the front, in plain view) and for some reason SIDS. It was awful.
I love the track titles on David Cross' albums. "Behavior That Would Seem Anti-Social Or Hostile Is Quirky and Neurotic When I Exhibit It Because I'm Jewish!" and "My Immigrant Mom Talks Funny".
What were the SIDS jokes? Paraphrase, please. I must picture this.