I'll be straight up and say SH nor RE ever scared me. I never liked RE that much, and SH2 was creepy, but it never got me.
Dead Space got me. Several times. The noises, the atmosphere, the childhood lullabies in the background, it all got me. At the time I was a Halo buff so training myself to aim away from the head took forever to override. Most of all, on Impossible it felt like I was supposed to die. The game made it feel like any necromorph could and would kill me, and every little victory was a success. When I completed the game I felt like I beat the odds, the way the game wanted you to feel. When something grabbed me I was afraid that it was going to kill me, I felt helpless, and the atmosphere made the sudden break in walking the narrow corridors so startling. Also seeing the suicides and maddening of the crew, with writing in blood on the walls, haunted me. And the noises... The noises.
DS2 kept the noises. And made the maddening of the Ishimura's crew translate to you. Watching Isaac, now a fully flushed out character, descend into madness was horrifying. The first game carried over, but now Isaac was at the forefront and I still felt so vulnerable... And the save system for hard core made me so anxious to cross into doorways. And the goddamn noises were still there.
All in all, atmosphere, insanity, difficulty in gameplay, gore, and music coupled with a rich story and overwhelming sense of dread and that the game does in fact want you to die. That's a good horror game.