Wow, you beat me, by almost a month, to that...I was going to mention Suspiria. Hell yes.
Some people (not sure whom) consider Battle Royale a horror film, so, if you're of THAT frame of mind, then I'll have to say that's my favorite, though I personally don't consider it horror.
Fallen? or is that more of suspense? probably.
Black Christmas...the opening scene of that movie STILL scares the everloving shit out of me.
I have to say, I was pleasantly surprised by The Grudge...I had thought it was going to suck, but I really liked it.
Invasion of the body snatchers, both versions. You could argue that it's more sci-fi, but the plot is enough to warrant 'horror'
And I LOVE the classics....Dracula, Frankenstein, The Wolf Man, great stuff.
Though nothing will ever top Psycho....I even loved the remake.