I haven't even been able to watch all of Avatar. It's very cliché, and the special effects have a cartoonish tint to them. I think Avatar probably gets universal agreement of being overrated in this thread. I disagree on Forrest Gump, but I will say it begins to fall apart once he gets out of the Army. Him being responsible for various pop culture icons during his run across the country was just overkill on the rest of his implausible journey. That being said, I still enjoyed the movie.
I thought American Sniper was just a rehash of the Hurt Locker. I didn't think it was a bad movie, but I can definitely see how you can square it not being a bad movie with the response it got being "overrated." Don't think the two concepts are mutually exclusive. Thought it was a B movie that gave you an interesting look into the life of a modern soldier. Not sure why it got Oscar praise. That is the problem with the movie.
Somebody mentioned Gone with the Wind. I think that movie is certainly overrated by today standards because it's more iconic to the generation it was released in. It was basically Old South nostalgia. There really isn't much of an Old South anymore, so the movie doesn't have the emotional influence/effect on today's people. I don't think as a book or movie that Gone with the Wind will be spoken about or discussed in 50 years.