I don't know about "most overrated" but "History of Violence" with Viggo Mortenson was one of the most disappointing movies I've seen in the last few years. I'd heard some good stuff about it before I went and was looking forward to it. It really, really sucked.
To me, lame movies with artistic pretensions are far more painful than lame movies that don't aspire to be anything more than enjoyable fluff. Thus, "You, Me and Dupree", or any movie with Jennifer Aniston or Sandra Bullock, cannot even get on the ballot for "most overrated" imho, even though they're the sort of movies I bitch about endlessly when my wife drags me to them.
Oh, and Basic Instinct 2 was really dreadful, too. Painfully bad. Never having seen the first one, I had no expectations and thus can't say it was overrated. But my wife had seen and liked the first one, and she thought #2 sucked too.