scary anime you ever watched

Mirrai Nikki, not really scary, but the girl is so frickin' creepy, I'm scared of it

Higurashi was really creepy for like the first 5 episodes. then it wasn't very scary. Same thing with Another

Last episode of School Days.

I wouldn't say I know a lot of horror anime but I know a good amount of manga that could creep you out.
Bio-Meat: Necter
Blue Heaven
Dorohedoro
Doubt
Fourteen
Gyo
Island
Parasite

If Corpse Party ever got an anime I could imagine it being really scary.

Higurashi is scary indeed, until the latter episodes.