In my family you have to watch the Godfather with my grandparents when you turn 15. It takes at least six hours because they stop the movie every three minutes to tell stories. Then a couple of weeks later you go through the whole shebang again with Godfather II. (Godfather III is exempt, however, as they hate it.) I love both those movies.
My family also has annual Scarface (the Pacino remake, I think) parties which they open with a big plastic chainsaw. I have actually never seen either Scarface thought.
The worse movie I ever saw, ever, was Finding Forrester. I still rue the money I spent on that one. I wasn't expecting much out of it though so I don't know if it really qualifies for this thread. Meet the Parents is a good one. Hated it. One movie that I think I was supposed to love and just didn't was The English Patient. Great actors, great story, and just eye-gougingly boring.