Horror doesn't work on me. Take that as a challenge if you like.

That said, I give the nod to The Ring (the American remake of Ringu). Overall I thought the movie was kinda dull, but I liked how the jump scares worked and looked. I didn't feel cheated by them, and the practical effects of making the girl look scared to death + seven days' water logged was downright freaky. I have a behind-the-scenes shot from the intro, when the camera zooms in on her and her face morphs. You can go frame by frame on the Blu-ray and that image isn't there, so I imagine makeup took the picture or someone else on set. You get more detail. Anyway, that image haunted me for years. I had it saved on my computer. (Still do in fact.) Now I look at it in awe, like "that's the image that got to me."

The scariest movie I watched was about child abuse, but that answer feels like a cop out for a horror community. Doesn't really matter which one. People being shitty to each other is what really gets under my skin.