Since we have a favorites thread, why not?
The following movies are so bad to me that at the time watched them I either felt like "I want those 2 hours of my life back!" or "I think I just became dumber from watching that."
Reindeer Games
Rollerball (the LL Cool J version)
Little Miss Sunshine
I don't remember the name but it was probably the first movie made by MTV. The trailers made it look like a comedy but there was nothing funny about it at all. All I remember was it was about a college kid who was a bout to flunk and discovered that there was a clause in the college rules that said if your roommate killed themselves you'd get an A in all your classes that semester. So he goes on the hunt to find a roommate so depressed that it just might happen. I felt like leaving during the first 10 minutes it was so bad. A lot of people did actually. But I had never walked out of a movie so I persevered. It hindsight, I should have left too.
Honorable mention goes to The Hangover. I didn't mind the movie itself but the pictures at the end credits scarred me for life and so I can't watch one of those movies again for fear I will see them again or something worse. .