The Exorcist
The Shining
Nosferatu
The Haunting (1963)
The Entity
The Thing
The Innocents

All are excellent horror/scary films, but, to me, The Exorcist every time!

Black Christmas(1974). It's not as utterly frightening as certain others, like "Halloween", but it's second-to-none with regard to atmosphere. The bone-chilling cold of the winter outside contrasts with the deceptively welcoming, rustic warmth of the sorority house...where a killer is lurking behind every doorway, grandfather clock, and coat stand. The groundbreaking cinematography and distinctively creepy music help create the aura of claustrophobic doom. I only wish the killer's location could have been kept secret from the audience until the decisive moment, as in "When a Stranger Calls", but then all those great early prowling P.O.V. shots would have needed to be nixed.

Torque. Oh, wait...this is about horror movies and not horrible movies that I love.

Some actual favorites:

The Texas Chainsaw Massacre (1974)
Halloween (1978)
Dawn of the Dead (1978)
Evil Dead 1/II
The Thing (Indeed a masterpiece.)
The Shining
Alien
Brain Damage (who knew anti-drug metaphors could be about brain-eating parasites?)
An American Werewolf in London
Prince of Darkness (so underrated)
The Fly (1986)
Videodrome
The Dead Zone
Dead Ringers
Re-Animator
The Lost Boys (the most entertaining 80's horror movie IMO)
The Serpent and the Rainbow
The Return of the Living Dead
From Beyond
Night of the Creeps
Friday the 13th Part VI: Jason Lives
A Nightmare on Elm Street
Planet Terror
Suspiria

Recently, The Descent and The Mist have made great impacts on me for the horror genre.

Some guilty pleasures in the genre too:

Silent Night, Deadly Night (arguably the funniest, least rational slasher movie ever)
Maximum Overdrive (great soundtrack, questionable merit...except it's so fucking funny)
Deadly Friend (Ma Fratelli and a basketball. The world was never the same.)

A few personal faves ino:
Black Christmas (org)
Phantasm
The Burning
Friday 1 and 2
Hell Night
Deadly Blessing
Wrong Turn 1 and 2
The Night Stalker

I recently ran into a guy I knew in high school 15 years ago and when he saw me he went 'RE-ANIMATOR!!!'. I asked him what he meant, and he said I lent him a copy of the movie on VHS and it totally fucked with his mind. He didn't remember my name, but he remembered all about the movie.
It was my favourite horror movie before that happened though.

Also,
The Others
Event Horizon
28 Days Later
The Thing
An American Werewolf in London

ps. my sig is from Re-Animator