So, eyah, just as I thought, mine has already been mentioned. IT.
IT is the first and to date LAST movie to have ever scared me. IT scared me so much as a kid that I became immune to being scared by a movie since. Of course, it also means I have a very slight, yet real, case of Coulrophobia. I can't stand clowns. I won't run screaming if I see one, but you'd have to wrestle me to the ground to make me get closer than I feel comfortable to one. That movie literally scarred me psychologically. For life.
Thank god the Coulrophobia was the only thing that stuck into adulthood, cause as a kid I was scared SHITLESS of the shower drain, too. And storm drains. And I was uneasy near red balloons, too.
Hell, to shower I had to take a Ninja Turtle action figure in with me. As a kid I reckoned, what better friend to defend you from a fucking kid-eating clown from hell that will pop up from the drain than a ninja turtle that LIVES in the sewers? Hey, that's sound kid-logic.