In a theater, or at home on DVD?
In a theater- probably Congo- saw it because I was a big Bruce Campbell fan and a big Michael Crichton fan... but the movie sucked and Bruce died in the first 3 minutes. Lame.
Honorable mention goes to Wing Commander, I saw it just to see the Star Wars ep 1 trailer. The trailer was good though, and as bad as Wing Commander was it was better than Congo.
The Thin Red Line is the only movie I have ever fallen asleep after watching it for an hour in the theater, slept for an hour, and woke up to find the movie still had like an hour left to go. The parts I was awake for was more interesting than Congo though, so it's not the worst movie... Though, since I missed the middle of the movie I guess it doesn't even count as a movie I sat through.
Indiana Jones 4 and The Happening were both very bad a disappointing, but they weren't nearly as bad as the above bad movies.
As far as at home, on Netflix:
Lust, Caution- another one that put me to sleep
Less Than Zero- aptly named, I think I was really bored that day otherwise I would have turned it off
The Godfather III- I only forced myself to watch it because the previous two were so good.