Worst movie I've ever seen was National Lampoon's Pledge This! I think what really made it worse is that Paris Hilton was playing a character that everyone thinks of her as... a high class prissy bitch and it was just annoying. The only redeeming factor was the hot chicks that they used.
Be Kind Rewind was also pretty bad. I mean, there aren't many movies that make me wish I never saw them, but definitely those two.
Most of the spoof movies are pretty bad too. The Comebacks was probably the worst out of all of them.