By bad, I mean movies that you know are awful (script, acting, plot, general production quality, all of the above) but you love them because of/in spite of it.
For me, it's gotta be High School Musical. I like two more than one (Bet On It, anybody?), but I do like them both... and I'm kinda stupid excited about the third one and how horrible/awesome it's going to be.
Stick It is a close second.