I have to agree with you on Tron 2. I was a huge fan of the original back in the early 80s and the sequel didn't get nearly enough respect from the critics.
My favorite underrated movie is Apocalypto. This film received only a 65% rating from the "experts" on Rotten Tomatoes, and flew under the radar mainly because of Hollywood's distaste for the director Mel Gibson. It is a wonderfully shot, action-packed movie set during the period of the Mayan civilization. A must watch for anyone who is a fan of good films.
Yeah apocalypto was really good!!