Horror movies are going to scare you if they're centered around your deepest fears. To me, Amityville was one of the scariest I've ever seen, and till this day it gives me the creeps when I see it. There's something truely evil about that story. I remember reading teh book, and weird things started happening when I had it in my house (no lie)
The scariest movie I've ever seen in Eraserhead. My biggest fear is of something so weird that has you trapped inside of their world and you can't get away from them. They're not looking to kill you, but to freak the hell out of you. the weirdness of them, and the close proximity you are to them, is highly disturbing. Eraserhead captured that fear perfectly.
i was watching "child's play" when i was five, so i'm fairly desensitized. when event horizon first came out in theaters, i got some frights. same with IT. and the american ring as well.
those are the only three that got me, even momentarily. frankly, selected episodes of the x-files have really spooked me as well.
The Exorcist: The version never seen (2000) was the scariest for me. Wanna see something really scary? Watch the The Exorcist intro, www.TheExorcist.net.