Horror films you feel are underrated.

As Dramatic Look Gopher puts it so well, you really should give the 1980 version (uncut) of Humanoids From The Deep a chance. Loved the creepy look of the mutated fish monsters and there is a fantastic portion of T&A, gore, memorable characters and all in all a really well made little monster b-movie, where you will notice a lot famous faces/names along the way.
I have not seen the 1996 (I think) remake, but if I come over a decent copy, I might give that one too a chance.