It would be 'The Exorcist' for me. I had waiting years to see it as it was banned in the UK. I was never able to get a pirate copy until I was about 17. I was still young enough to get scared then and I still had some remnants of my catholic upbringing to affect me. I know a lot of people hate the long winded beginning in Iraq but that's why it worked so well for me. It gripped me from the start and unnerved me throughout. It stayed with me for days. The idea of being attacked and manipulated by an invisible entity you had no way of fighting was horrifying. Now I am older and don't believe in possession, it doesn't have the same impact but I still love it. The biggest scare was The Exorcist III. I jumped out of my skin. Special mention to the head popping out of the boat in Jaws.