For whatever reason, I was allowed to watch The Amityville Horror at the tender age of 9 (I think my parents weren't home, and my brother was watching it). Despite being on TV and heavily edited, it gave me nightmares for the next week and made me terrified to walk down the basement stairs. There have never really been any other movies that creeped me out to such an extent.
I remember hiding behind the sofa as a kid when watching Dr Who. Three stand out in my memory. The worst were the invisible spiders, interesting that the things you can't see are the scariest. The other two were the giant maggots and the Egyptian mummies.
Oh, snap! Obviously I have so successfully repressed that memory that I am in total denial. Living on Florida's right coast, where sharks regularly cruise just off-shore, I still get spooked when I swim out any distance from the beach. I really do start to hear the music well up, da dum da dum da dum da dum da dum da dum da dum da dum dum dum dum dum dum dum dum dum dum dum dum dum dum dum da da dum deedly di deedly di deedly di deedly di deedly DI!
Great choices and I'm glad that so many here are well versed in the darker side of cinema! I love all of the classic old Universal films and the Hammer Studio films with Christopher Lee, Peter Cushing, and their signature line of buxom maidens.
Jaws and The Omen both scared me, but my favorite fear film is "Angel Heart", starring Mickey Rourke and DeNiro. Yowsa, I felt like a needed a couple of stiff drinks when I walked out of the theatre!