The Room. 
I'm not gonna defend that movie in any way, but as far as so-bad-it's-good experiences go, it's a real treat. 
Oh man, The Room just takes me back to my sixth form days. I'd invite a few friends over for a sleepover with pizza and we'd always get drunk and watch/laugh at The Room. Fun times.
So of course there are the LBT sequels. Jurassic Park III is a guilty pleasure as well.
For some reason, I have a lot of Christmas-related guilty pleasures, including Last Christmas, The Holiday, and Love Actually (and I'm usually not a fan of rom-coms

).