I've watched a lot of bad films, but the #1 spot has to go to D.E.B.S. That film is god awful for the first fifteen minutes until you realize the angle its playing, whereupon it becomes incredibly awesome.
I'm going to have to disagree with a lot of people in this thread though, some of the films listed here aren't bad at all, they're pure win. The biggest case of this would be Shoot'em Up which is just pure kickass distilled into movie form. Kung Pow: Enter the Fist was always a parody, but that movie grows on you after you finish it so that the twentieth time you make a joke referencing a scene from it, it's funnier than when you saw it originally. I also give the benefit of the doubt to Eurotrip because while the entire genre it's part of (teen comedies) is mostly retarded, that film has some real knockout lines ("Bratislava", "worst twins ever", "scusi!").