It's got to be The Conjuring 1 & 2 for me. It scared me so much that I literally couldn't muster the courage to walk back to my dorm room alone because I watched it in my friend's room around 1am in the morning.

The scene were Lorraine is hanging clothes then breeze blows a white sheet away and then it hangs in the air for a moment briefly showing the figure of a woman scared me so much that I literally removed the sheets from my bed that night...wasn't going to get caught by any sheet possessing ghost.

The sequel with the demon that possessed a ghost that possesed a girl was just something else!!! The demon in that one scared me so much that till this day I haven't been able to bring myself to say the demons name out loud, I mean like I'm Catholic so you can understand why a demon dressed up like a nun would scare the living shit out of me.

I've long since sworn of watching dark and scary movies though, I don't have the heart or mental strength to deal with them anymore.