Boy, where do I start.
No way I could list just one.
So here I go...
The scariest movie I saw as a pre-teen: "Legend of Boggy Creek". Saw it at the local theater as a new release. Scared the living bejesus out of me.
Early teenage– "Jaws." I was 14 at the time. Make an incredible impact on me.
Late teenage– "Dawn of the the Dead". The original. Fudged my learner's permit card so that I could get it to see it. The film was rated X for violence and no one under 18 could get it. Well, I was 18 for one night.
An amazing film. Couldn't believe how scary it was and how incredible the special effects were (at the time).
Went to see a ton of mostly slasher films from about 1980-'85. "Friday 13th", "Fun House", "My Bloody Valentine", "The Shining", "The Fog", "Hell Night", "The Things"... All good films, some better than others.
Over the last decade or more horror films haven't made as much of an impact on me although I've found "Cloverfield", "The Mist", "Jeepers Creepers", "Abominable" and "Quarantine" to be good flicks.
Back in the early to mid-70's there was for a time "the made for TV movie" which featured some very good horror films, such as:
"The Night Stalker" (the movie)
"Don't Be Afraid of the DarK"
"Bad Ronald"
"Duel"
"Killdozer"
All are highly recommended for the serious scary movie connoisseur.