Some good picks here.
I can't back Cabin in the Woods for a second. I get what it was trying to do. I think it beats you over the head with what it was trying to do but I found the idea really dumb. The self aware, deconstruction of really obvious horror tropes didn't impress me at all. I hate the Scream series for similar reasons.
I can't recommend Martyrs enough. That movie delights in fucking with you and throwing you for a loop with every twist and turn.
Return of the Living Dead
Night, Dawn and Day of the Dead
V/H/S
The Fly
Zombie 2/ Zombi/ Zombie Flesh Eaters (This damn thing has a lot of alternate titles) It features a Zombie fighting a shark.
Ti West is doing some great work. His movies are a really burn but he actually gives a shit and puts time and thought into his characters.
The Exorcist of course but do yourself a favor and check out The Exorcist 3 as well.
Pieces
The Descent
REC
The Sentinel
Black Christmas
House on Haunted Hill (the original)
Sleepaway Camp (If only for the ending)
The Burbs (Its certainly not a horror film but it has a light hearted halloween feel)
Anyone seen Antichrist with Willem Dafoe? That movie had some bizarre shit in it. And the sex scene at the beginning had full penetration haha.
I believe that was a stunt cock.