Evil Dead? 28 Days Later? Huh...two of the most talked about horror films ever, the former considered a massive cult classic, the latter often held up as a example of the modern horror genre, not -exactly- underrated or unknown, still, enough kvetching here's my pick:
The Re-Animator series, Re-Animator, Bride of Re-Animator and Beyond Re-Animator (Beyond isn't my favorite and is the worst of the series imo, but is still pretty competent.) A excellent set of movies ruled by the awesome Jeffrey Combs as Herbert West, loosely based on the stories by H.P. Lovecraft.
The Phantasm series. Cheesy B-Movie horror about two friends and their battle against the sinister 'Tall Man' who's stealing corpses from the local morgue for his own evil ends... The first movie's a real treat, but I wouldn't actually recommend the rest unless you really get into it.
Fright Night. Okay...yeah, this movie just screams '80s vampire flick', but it deftly handles many of the old vampire myths and it's got a pretty damn good vamp for a villain as far as Mr.Danderige goes.
For more of an anthology, Creepshow is the film to go for. Directed by George A. Romero (of Night of the Living Dead fame) and written by Steven King, it's a series of five short horror films. While they do vary in quality it's not hard to find one you'll like out of the bunch. My own favorite being the story about the reclusive bastard who's home get's invaded by murderous cockroaches.
