Requiem for a Dream is one of the most depressing things I've ever watched, and I refuse to see it again. Not a bad film, but it puts me on a bad spin.
Battleship Potemkin, I never want to see again, simply due to... well, five times is four times too many. (I had to write uni essays on it's use of montage theory, it's context in film, reference it in a essay about the mythology of the October 1917 revolution, it was on outside the interviews before I ever came to uni, and I watched it independently before that due to an interest in Russian history. Just read Eisenstein's writings on montage theory, they're much more interesting than watching any of his films more than once.)
Titanic, Only Yesterday, Resident Evil, and The Outlaw Josey Wales - I just remember as being unspeakably dull. Same story with slashers/the typical summer horror movie.
Gore films plain disgust me.