I guess my first disturbing movie was 'Squirm' in 1976. It was about (lol) a power line that hit the ground and caused all the worms in the ground to become flesh eaters, so they did that. They ate people. It's a joke now, but at 10 or 11 years old it freaked me right the hell out. Then, as I got a little older, that movie became stupid and was replaced by The Exorcist. The Exorcist came out earlier, of course, but I wasn't allowed to see that. I saw it when I was 13 - somewhere - and it caused me (then) irreparable harm. I barely heard curse words back then, let alone some of the sentences contained in that one (in addition to the crucifix scene). Literally mouth agape reaction when watching that one. I think that one, Jaws, and The Omen were the ones that sat with me the longest. Burnt Offerings was another one in the 70's that left me feeling uneasy.
In the 80's it was Motel Hell, Hellraiser, Re-Animator, and Henry: Portrait of a Serial Killer.
Every movie you get through makes you more immune to being freaked out over time, so it becomes increasingly harder to 'disturb' me now. I'm not looking for gore, because that can't get any worse. I'm looking to get mind-fucked now. The first SAW was good like that, but they had to keep trying to 'improve' upon it (dumb). Movies like Silence of the Lambs, "We Are What We Are" (that movie sneaks right up on you at the end), The Sixth Sense, Se7en, Jacob's Ladder, Disturbia, Identity, Orphan, etc., are great - initially - for that 'disturbed' outcome. I like movies where you really can't predict where it's going or how it'll end.
Honorable Mention: 28 Days Later. The first time Zombies were able to sprint? GTFO.