Scariest horror movie you've ever seen??

Seven

Growing up I was TERRIFIED of anything remotely scary, creepy or weird in movies. It's actually to that that I attribute my total fascination with horror and the bizarre as an adult.

I remember as a kid seeing the cover of Hellraiser at the video store and thinking that surely anyone who watched this movie would literally DIE it was so terrifying. I could not even handle Beetlejuice at that point.
As a young teenager I started deliberately exposing myself to everything purported to be the scariest, creepiest and weirdest, and over time came to love all of it. So most of my scariest horror movie experiences were things like seeing The Shining and Texas Chainsaw at a much too young age.

I'm mostly desensitized to movie induced fear now, but there are certain things that in the right situation can stimulate my hyperactive imagination and get me paranoid. For example, stuff about aliens can for some reason be touchy, The Shining can still get me legitimately creeped if I'm alone, interestingly enough A Nightmare on Elm Street, one of my all time fave movies, in spite of being relatively goofy and fantastical, really gets me in a vulnerable place. I saw a documentary on sleep paralysis a few years back called something like The Nightmare that I found very scary, and even moreso due to how it could connect to ANoES.

It Came From Beneath the Sea.

I was surprised at how much It Follows 2014 creeped me out.

Is The Silence of the Lambs considered horror? I'm not a horror fan but that's the one movie I saw I wished I hadn't because it was so terrifying.

The original The Innocents from 1961 was scary. So was the original The Haunting. The Haunting was very effective for me for the same reasons The Entity was for you -- you don't see anything, but the music, the atmosphere, and your own imagination are what make it.

No matter how silly the movie, Chucky of Child's Play legit scared me when I was a child.
Any body-morphing gore especially from The Thing also scared me. It felt so real to me before.

Ghost Story (1981). The trailer, paired with the film's trite-sounding title, was misleading. A bunch of old guys sitting around telling scary stories...? Big deal, or so I thought.

I found the manner in which the woman in the story dies to be extremely horrifying. Drowning in a slowly-sinking automobile while the men watched helplessly from the shore, unable to intervene, was very disturbing to me.

Signs.