First of all, those of you saying Forrest Gump is overrated need to get your head examined.
Gonna break the rules a little bit and do more than one.
Overrated (in no particular order):
The Big Lebowski: Yawn fest
Rocky Horror Picture Show: I know it didn't do well initially and became a cult classic, but it doesn't deserve to be a cult classic.
Avatar: Too long. Too uninteresting.
Star Trek, The Wrath of Khan: It's the only movie I ever walked out on. By far the worst Star Trek anything.
Underrated (in no particular order):
Punch Drunk Love: Adam Sandler is amazing in this non-typical Sandler movie. The whole movie and cast is great.
Napoleon Dynamite: Yes, a lot of people love it, but it's not in their top 5 movies of all time, and that's where it deserves to be, so underrated.
Crazy People: A good comedy starring Dudley Moore. Underrated because people give it bad ratings, but it deserves much better. Has some very funny moments.
Shattered: A well done mystery.