FanFiltration said:

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.