Apocalypse Now. I've never seen any movie that I couldn't sit through, even though I've seen a few pretty bad ones. I actually liked Twilight though, so maybe that says something about me...
Anyway, I studied Joseph Conrad's novel Heart of Darkness at A-Level (set in the African jungle), and so as part of an English film group that our English teacher had set up (mainly supposed to be showing films related to texts we were studying, although Apocalypse Now was the only film that was ever shown) a small group of us went after school to see the film in one of the empty classrooms. Time constraints meant we couldn't finish it, and when we were asked if we wnated to see the rest at another time we all said no. General consensus was that it was crap. Ironically enough, we never reached the bits with Marlon Brando, and we'd previously seen A Streetcar Named Desire for our AS-Level set text, which also starred him in a leading role...