Freaks (1932) - A classic horror film that stands out for its question of what defines human and inhuman, in a period where most horror films were about fantastical monsters.
Citizen Kane (1941) - It's a great film. Is it the greatest film of all time? No, and I don't believe you can determine one film as the greatest ever. It's great just for having some of the most gorgeous cinematography to ever be put on film.
House of Wax (1953) - One of Vincent Price's best.
The Haunting (1963) - A great horror film about suspense, atmosphere, and the scariness of what you don't see.
An American Werewolf in London (1981) - A great mix of horror and comedy; the transformation scene is still impressive.
The Evil Dead (1981) - Another great mix of horror and comedy, albeit more in a more gross out vein. You can tell from this early film that Raimi has talent.
Re-Animator (1985) - I didn't even notice it, but I was really on a horror-comedy roll. Jeffrey Combs is perfect as Herbert West and the film goes from crazy to insane near the end.
Jacob's Ladder (1990) - A brilliant film about war and religion. The party scene in particular is a mindfuck.