I've always thought movies like 'Joshua Tree' with Dolf whatshisface were the dumbest. Impossible gunfights, badly overacted bad guys, implausible sex (which is really OK but usually it doesn't make sense in the movie).
Another vote for Titanic.
The second American Pie wasn't very good at all, but decent enough to avoid (barely) my list.
Me Myself and Irene
Pretty much anything with Jim Carrey.