I listen to the comedy channel on Slacker Radio and find just about all comics funny in short bursts, but I will almost always skip...
Adam Sandler - especially when he's making stupid voices. His few shining moments were on SNL when he sung things like Lunch Lady Land and the the Hanukkah Song.
Any comic trying to do comedy music. Very, very few comedy songs are actually funny.
The stupid prank phone call bit. Crank Yankers was funny about 60% of the time. Nobody else should even try.
Sarah Silverman. I used to dislike a lot of female comics, but for some reason the ones I've been listening to lately seem to be pretty funny. I can't say the same for Sarah.
Any comic that has poor timing. A few seconds off dead air and poor timing can kill otherwise good material.
Lately I've been listening to a lot of Dave Attell and revisiting one of my favorites Doug Stanhope. Stand up comedy is just fun.
And Fluffy haters gonna hate.