For all things scary.
It was the first horror movie I saw, tout court. The blair witch project.
I was 12 years old. Where previously my mom would hire a babysit for the two of us, this was the first time home alone with big sis babysitting.
She completely convinced me it was "found footage".
Ugh! I saw it the night it came out. Also believed it was real found footage, as the ads sold it that way. Spent the last thirty minutes scream-crying into my girlfriend. Stumbled out of the theater by the fire exit afterwards... Into to the fucking forest behind the place! Definitely ranks up towards the top of the most insane fear experiences for me.
Other people who saw it that first night found little stick dolls on their windshields that really freaked them out.
I saw IT when I was a little kid, and it messed me up because I misunderstood the plot. Because it shows the kids fighting an evil clown, then 30 years later, returning home to fight it again, I somehow got it into my head that IT was an immortal being that would always return every 30 years, no matter what.
The futility of fighting what is essentially an unkillable god really freaked me out. You can never defeat it, only push it back into hibernation for a little longer.
Somehow, though, it never made me scared of clowns, only of Pennywise. I actually love clowns, both regular circus clowns and horror clowns.
Friday the 13th in 3D. Saw it in the theater when I was apparently 12, with a whole junior high friend group IIRC. Don't know what we or our parents were thinking, sure left an impression and a couple of night's worth of bad dreams. The 3D effects were effective and pretty nasty.
Event horizon
Watched The Thing waaaay too young.