I think bad movies are just as fun to watch with friends as good movies are. I've hosted a lot of bad movie nights:
Xanadu
Can't Stop the Music
Grease 2
Towering Inferno
Supergirl
Poseidon Adventure (original)
Aiport, Airport 77, etc
Rollerball (original)
Catwoman
I think the lesson here is that musicals, disaster movies, and female superheroes are often bad. I know I'm forgetting some, as well.
Anyone have any good suggestions? It's getting harder to think of new ones.