I know some people are going to take offense to this because I'm defiling their sacred cow, but: The Breakfast Club. I'm sure I could think of something with worse acting, but a lot of people give this one a free pass. Was The Breakfast Club well written? Check. Horrible 80's and Teen-Movie Clichés? Check. Too bad they forgot the actors. The frelling janitor is the best actor in the goddamn movie, and that's saying something. They keep on stepping on each others lines and come off vaguely wooden.
Also, Titanic. If I want to spend 4 hours watching a damn movie, there better be something in it for me other then a damn boat a splode. Both Kate Winslet and Leonardo DiCaprio are HORRIBLE actors. A goofy accent does not make you an actor. There were some decent actors on staff, like Kathy Bates, but none of them showed up enough to make a difference.