I'm sticking with 2 Live Crew because it's just so offensive as well as lacking in musical quality.
Beyond that, I've never really gone for the gimmicky songs. Somebody already got Who Let the Dogs Out. You've got stuff like Macarena, Gangnam Style, I'm sure there are others I'm mercifully forgetting.
American Idol foisted William Hung on the world. That seemed like a cruel joke to me, not people actually supporting him.
The worst live music I ever heard, I don't even know the artist's name. It was some absolute nobody that for God knows what reason was the opening act for Jenny Lewis at a show in Seattle. It was HORRID. I felt personally insulted, I was insulted on Jenny's behalf, it was mind numbing how bad it was. I cannot believe a tour manager or executive or whoever actually booked that guy to open a major show. Beyond ridiculous. How bad was it? I nearly went blind temporarily, that's how bad. I'm serious. Now it was probably just my contacts sliding out of position on my eyeball, but my vision went blurry, and it returned to normal after he left the stage. I have to draw my own conclusion about that.