Underrated films that I enjoy as much as (and sometimes more then) the original. All of these films got some bad press, and disappointed a fair amount of moviegoers'.
The Exorcist III
The Two Jakes
The Trial of Billy Jack
2010 The Year We Make Contact
More American Graffiti
French Connection II
The Four Musketeers*
*did not get bad reviews, but did cause a great deal of controversy.
Fun fact: the guy who plays Cindy Williams' brother in More American Graffiti (Will Seltzer) was the runner-up for the role of Luke Skywalker.
On topic: totally agree with Star Trek III being underrated.