Hi! Interesting question!
I'm one of the oldsters here, so most of the scary movies I've seen are pretty old (and tame), compared to more recent films. Personally, I like a scary movie with a good story, so I rarely watch anything that's just made to scare the bejeebers out of me.
Vampires, werewolves, and zombies don't really scare me. Ghosts and haunted house movies are fun. I like gothic horror, but don't really find it scary.
Almost anything by Stephen King is creepy. Clowns, dolls, creepy kids, and exorcisms freak me out. I've never seen The Exorcist all the way through and probably never will. I hate slasher films and do my best to avoid them.
Poltergeist, Invasion of the Body Snatchers (1978), Rosemary's Baby (the book is better), Alien, 30 Days of Night, and Crimson Peak are all favorites. Carrie scared the crap out of me as a teenager and was one of the first scary movies I saw at the theater.
But to answer the original question, I'd have to say Texas Chainsaw Massacre - the original. Saw it at the drive-in as a teenager because a former boyfriend insisted he had to see it.

Once was enough for me. Never again. Possibly why I hate slasher films so much!
And I have probably now dated myself.
