I'm going to assume you've seen most of the high profile classics, as well as the popular stuff the 80's, so I won't waste your time with that.
Here are some I've seen that are a little more obscure:
Brain Damage - Very bizarre one about a parasite that eats brains via a host whom it manipulates by asking politely.
Not messing with you, that's what happens.
Basket Case - A little more well known (and from the same director as Brain Damage), it's a about a dude who may or may not be a serial killer who carries a basket everywhere he goes, what's in the basket? Well you just have to see it.
Terrorvision - this one is less of a straight horror movie and more of a horror comedy, it's about a monster/alien/thing that gets teleported into a suburban home, that sounds like the premise for a family movie but trust me, it is definitely not. Basically it's "the 80's: The movie," and if that sounds like your kinda thing, you're going to like it.
The Cabin in the Woods - You may or may not have heard of this movie, but if you havn't then all I can say is this: DO NOT LOOK IT UP, JUST WATCH IT. Do not Google it, do not do an Imdb search, do not ask anyone what it's about, just see the movie.
You'll thank me later.
Alice Sweet Alice - I don't like this movie, but I'm pretty much wrong. It's a slasher/mystery movie with a heavy religious theme, it's genuinely disturbing and has an atmosphere about it that just feels... wrong. Personally I think the twist at the end is really fucking stupid, but allot of other people seem to really like it so judge for yourself.
The Return of the Living Dead - I had assumed that this movie was common knowledge umong horror fans, but I'm seeing more and more people nowadays who don't know it, if that's the case with you, then you need to see this movie. It's the perfect balance between horror and comedy, and honestly I think I'd call it my favorite non-George Romero zombie movie.
And of course the obvious stuff, Halloween 1-4, The Evil Deads (and Army of Darkness, even though it's not really a horror movie per say), anything by John Carpenter, Nightbreed, Re-Animator, etc. etc.