I'd have to say Avril Levegne (or whatever the hell), just because, as Flamethrower pointed out, there's no way Green Day is worse than her.

Barry Manilow. I mean, c'mon, y'all missed this?
Cher. See above.
System of a Down. SOME of their music is decent, but they make up for it by being, well, stupid.
Slipknot. There's no way I'll give my money to people that try to make themselves look that dumb.
ICP. See above two.
Nirvana. Face it, Kurt Kerbang really was a loser. And so are you.
Marilyn Manson. Do I really need to explain this?

There's eight. Maybe I'll think of a couple more.