In general I think most horror movies that are any good get way overrated, but be that as it may, there are a few that don't get a good look as often as they should in horror movie discussion:

The Evil Dead - okay, it's recognized as launching a mini-revolution in horror, it's Sam Raimi, and it's a cult classic, but this is a seriously under-mentioned flick in horror discussions. Blair Witch meets the Exorcist meets Scream.

Scream - okay, again, Scream is a movie recognized in horror circles as good or even a classic. You'll find it on many "Top Ten of Horror" lists, but it's too often ranked behind worse movies. It's a slasher flick that does it better than 99% of its genre-mates including ALL of the Halloweens and most of the Freddy and Jason movies, and laughs at them, and itself, while doing its thing.

The Birds - Perhaps it doesn't bear the charm of Psycho, but it's easier to relate to...most of us will never encounter a Norman Bates, but the birds, "the birds are everywhere!!!"

While I can't find many that agree with me on this one, the original Children of the Corn should be a horror classic.

There are some movies rarely classified as horror, but which never the less induce psychological trauma that rivals anything any horror movie does:
Mulholland Drive
Donnie Darko
Requiem for a Dream

Special Note about Night of the Living Dead:
I just watched this movie for the first time, and I think it barely fits in the horror genre anymore (I believe the same could be said for the early Dracula and Frankenstein movies), but in my mind it belongs in the discussion of the greatest movies of all time. Not top ten or twenty, but top fifty maybe, and certainly top hundred. It's a shame that being an on-the-cheap B-horror movie keeps it from being appreciated for what it really is...a non-sensationalized (relative to 99% of movies anyway), albeit stereotypical look at how people respond in the face of a very immediate, horrible, life-threatening crisis, where the crisis chosen just happens to be the sensationalized zombie menace. It's also a veritable handbook for how to create suspense with acting and setting alone...IOW if you do it right, you don't need to keep the audience wondering where and when the bad guys are going to pop up to keep the audience believing that the main characters are in imminent danger.

Anyway, now back to the topic...