I'm going to break it down a bit.
Scariest for "jump scare" factorThe first Resident Evil in all it's cheesy glory scared me numerous times. Those damn Dobermans jumping through the windows behind you!
I must have jumped out of my chair a dozen times playing this game!
Scariest for visceral "fight or flight" factorThe first Dead Space. After fighting off mutated beasts coming at you from every angle, I found myself stomping corpses well past the point where they were already dead. I would literally become locked into the terror.
Scariest for "psychological" aka F*ing with your mind factorI have to give it to Silent Hill 2. I still have nightmares of "Pyramid Head" ravaging some manequins.