Im goin to have to say first of all:
The Ring - the re-done american version, i loved it, saw it in the cinema, and it genuinely scared me. Original also scary.
Blair witch is up there, as when it first came out so long ago (remember?) there was serious talk that it was ACTUALLY REAL!
hmmmm... how about the first Childs Play ? it did scare me a bit...i was kinda young dudes seeing a doll runnin around like that can scar u for life !!!