The nightmare on elm street series was so scary for me. The fact that he can get you when you're sleeping just makes you feel so helpless.

Well, I usually prefer asian scary movies, but I suppose that I liked the Poltergeist movie and Quarantine.
Quarantine was interesting because it was almost like you were there with them during the movie, so I really enjoyed it. (I've heard the original spanish version is ten times scarier though)

And, though I know you have this under American films and whatnot, I just thought i would mention that Two Sisters (korean) is pretty scary and so is the original Grudge.

yess! Two Sisters is amazing and the remake (The Univited) is useless and not scary!

All of the Grudge movies scared me, japanese AND american

Freddy Krueger is a personal favourite of mine,
and anything by Stephen King is usually terrifying.

One part in his movie Pet Sematary, I can't even watch because the character scares me so much, is the sister with spinal meningitis, her name is Zelda , I get terrified of even her voice!

me... the scariest one is THE EYE (Japanese/Chinese)... and it is also good movie..