I've actually witnessed Larry the Cable Guy being funny. He was on the Tonight Show and was somewhat cleaner than usual. Also, he had to keep up with Jay Leno. Whether or not you think Jay is funny, his routine is at least not entirely reliant on fat and toilet jokes.

I wouldn't say he was the worst comedian ever, but we had a comedian come to our small, left-leaning, liberal arts college when I was in school. He went through his usual schtick of girlfriend and boob-job jokes and was greeted with cold silence. He finally stopped the show, admitted that he'd never bombed this badly with a college audience (and he really was bombing spectacularly), asked what was up, and then made a valiant effort to use gags that were better suited to his feminist/tree-hugging/gay-sympathizing audience. He was much funnier without the boob jokes.

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The best comedian I ever saw, on the other hand, was Elvira Kurt. She came for Gay Pride Week and I almost didn't go because I figured I wouldn't get a lot of lesbian jokes (since I'm not a lesbian). As it turned out, there weren't that many lesbian jokes and I laughed until I was in pain. And then kept laughing. She nearly killed us.