What is the scariest movie you have ever seen?

When I was small I watched all the movies of Child's Play and all of them are very scary, except the last one when it started to be funny because the doll had a family and acted like human being. They argue just like husband and wife and the scare vanished just like that because of the funny dialog. As an adult, I watch Ju On a Japan horror movie. It was the scariest movie so far to me. I can't sleep without checking under my blanket for three weeks. Many times I had to purposely check under the table, in the closet and when switching on the television I was somewhat hesitant. Lol.

I am on a perpetual quest to find a horror movie that keeps me awake or gives me nightmares.... Strange quest, I know but everyone needs a goal, right? The only movie that has ever given me nightmares was Titanic because I nearly drown as a kid and am now afraid of it. It's not a crippling fear but after seeing Titanic I had some very strange nightmares, all centered around drowning. As for actual horror movies... I have been very impressed by M. Night Shaymalan's movies - The Sixth Sense and The Happening, in "horror" - although I thought The Happening needed a little more.... something. The ending (prior to the final five minutes) was a major let down. Halloween 1, 2 and 4 were good. After that, it started to be rehashing the same movie over and over again. Seven freaked me out a little. I always get a little weirded out thinking about the reality that people like John Doe (Kevin Spacey's character) really exist in the world and while they have a complete and thorough rationale for what they are doing, you never know when you might slip up and do something that makes you suddenly fit their profile. Freaky stuff.