I'd have to say The Omen & The shining. oh and the original Dark Water.
I don't watch that many horror films, they don't bore me i'd just rather often watch other stuff.
As a kid i saw Poltergeist and i had to sleep with the stairs light on for years afterwards as it scared the living daylights out of me, there used to be a tree at the bottom of our garden and that didn't help me at night as i kept thinking it was going to come get me.
Ring - all 3 originals in fact - fuuucccckkkk
Halloween - doesnt need dark to be scary
The Fog (some bits are perfect shiver material)
Psycho
Cronos
Prince Of Darkness
However I also rate Scream and American Werewolf in London - so clever
Let's not forget Shaun of the Dead - awesome comedy horror
Agree with Carey on guiltily liking Ginger Snaps. Other than that it's all the Evil Dead films, Dawn of the Dead, From Dusk til Dawn, Planet Terror, and generally any horror that doesn't take itself too seriously.