I'm not a horror fan, but even I like Alien and The Descent - so maybe those? There's a similar thread here, if you're interested!
i havent seen too many movies but i do like horror! one of my favorites is the mist, and also night of the living dead. world war z is pretty good too, and one of my all-time favorite movies is american pyscho, though i might class it with psychological thriller rather than horror. and of course, silence of the lambs and the shining are great too, though i prefer the book for the shining!
I'm a HUGE fan of horror movies, every subgenres. Some of favorites are The Shining, Suspiria, Alien, Videodrome, and Texas Chainsaw. There's so many fantastic movies out there. I'm particularly a fan of Argento and Cronenberg as directors/writers.
I would suggest Seventh Moon. And if you're into horror comedy similar to Shaun of the Dead, then try out Tucker and Dale vs. Evil.
Have you seen many Japanese horror films? Like the "Tomie" series? Or "Ringu?" Or "Juon?" It's not strictly horror... it's kind of a sicko musical... but there's "The Happiness of the Katakuris." It's really creepy and funny. If you go on Youtube there's this awesome series of videos called "Tales of Terror from Tokyo." Some of those are simply weird or moody but some are genuinely disturbing. And "Ghost Stories from Japan." Those are really cool, too.