From a "whuh?" angle, Black Swan was hard to watch. So much of the movie I found myself asking, "What's the point of this?" Also, there's more than enough WTF? factor to screw with your head, but unlike movies like FIGHT CLUB (which have you in WTF? mode for the majority, but then everything clicks together in the end), by the end of Black Swan I was still asking, "What was the point?"
From a "wincing 'oh I didn't need to see that...'" angle, Requiem for a Dream. Those of you who've seen it know what I'm talking about. I spent the rest of the night feeling as though my mind had been violated. No movie has disturbed me more.
From a "this is just...bad..." angle, I have two that come to mind. The first is Rush Hour 3. I liked the first two Rush Hour movies, but I stopped watching the third about 10 minutes in. The acting and the writing just seemed so forced...like they shot the whole thing over a weekend and everyone was allowed only one successful take before they moved on the next scene.
The second is Tristan and Isolde. As a married man, I've watched more than my share of chick flicks, but this one was just bottom-of-the-barrel bad. It was a bastardization of Shakespeare obviously designed to appeal to 13-year-old girls who like bad romance drama. We were, no joke, 20 minutes away from the ending when my wife said, "Do you even care how this ends?" This stands as the only movie that I have turned off after watching more than half of the way through.