The original Night of The Living Dead
I'm a fantasy and scifi horror fan. My main go to in a pinch is always the Necronomicon, Hellraiser and Wishmaster series with the original Pinhead and Wishmaster not the new ones. The runners up would be have to Event Horizon, Sunshine, and of course the original Evil Dead series.
As a child, I was scared by THE BLOB. The idea of a creature that was immune to harm, had a bottomless hunger, and couldn't be kept out by locks or doors was more frightening than just about anything else I could imagine.
"The Thing" is the first one that comes to mind when i am asked this question.
However the "Trilogy of Terror" was my earliest memory of being really scared at a movie.
that was a good one!
I think that Blair Witch scared me most
I love The Sixth Sense. Not sure if it's considered a horror movie, though.