Gotta say...The Exorcist. I was 11 and sneaked in to see it (I would have NEVER been allowed to see it by my parents). I could barely watch it. I had nightmares for a week and I swore I would never watch a scary movie ever again. I didn't see another one until I was 18.
Nightmare on Elm Street. Freddy Krueger is the ultimate in horror characters. Damn, I still remember sneaking out of bed to watch it and then not sleeping for like 2 days.
Eraserhead was like letting David Lynch take a scredriver and make bad adjustments between my ears. Only saw it once - that was plenty. Not scary, just so freaking weird.
Y'all are pretty young. The one that got me going was "Wait until Dark", a 1967 movie I saw when I was in HS. Even with 'Nam, things were different back then regarding suspense/horror flicks.
And the leprechaun movies are gay, but they still scare me lol. AMC is running all the old scary movies from back in the day lately. Needless to say, I've been sleepless a lot lately!