One of the key difficulties in asking a question like this — and it’s not something you’d know unless you spent time seriously analyzing the genre — is that there isn’t, for lack of a better way to put it, “one kind of scary.”
In one of his books on writing, Stephen King described something I’ll call the “monster behind the door” problem. If the storyteller gives you a character approaching a door, and suggests there’s a terrifying monster on the other side, he says your imagination begins speculating on the nature of the monster and why it’s so scary. And the thing is, your imagination is entirely your own, and will come up with a monster that frightens you, specifically. But then when the door is opened, the monster has to be shown (or described), and maybe the storyteller’s monster isn’t scary to you any more. “Oh, it’s a ten-foot creature? It could have been a fifteen foot creature.” Or whatever.
He says one of the ways around this is to minimize the specific detail (unless you’re confident you’ve got a really scary monster). Lovecraft, for one example, does this a lot, describing the monster as something the rational mind can’t grasp, but including hints and suggestions to get your imagination to work against you. Or maybe don’t open the door at all, leaving the creature entirely suggested, fully in your imagination.
However, the more I read and watch horror and think about different people’s reactions to it, the more I believe King’s analysis here is incomplete. Because the reality is that some people’s imaginations don’t work like that. In some ways, it feels like some people don’t have that kind of imagination, at all. If you show a closed door and say there’s something scary behind it, some people will be scared, whether or not you open the door. But other people will just see the closed door, and will be unable to visualize anything on the other side; and if you don’t open the door and show them the monster, they’ll be disappointed, and say the story sucks.
A good example of this is the movie The Blair Witch Project (the original from 1999). It’s a movie that works entirely on suggestion and implication. It doesn’t show you anything.
I have a very active imagination, and this scared me out of my wits. But I also have a friend who’s the lacking-imagination type, and he thought the movie was lame, and not scary at all. As far as he was concerned, there were three dumb kids in the woods, and some noises, and that was all. The movie didn’t show a scary witch stabbing someone. All it showed was three characters, some trees, and an old house. For him, not scary at all, because there was nothing scary shown. For me, very scary, because of what wasn’t shown.
All of which means there isn’t a single objective “most scary” movie, even in a theoretical sense, because no matter how scary it is, it’ll be scary only for part of the audience.
My older daughter is 14, and is very interested in horror movies. I’ve been carefully showing her a mix of classics (the original Night of the Living Dead) and modern-era horror (the American Ring), not just to find the limits of her tolerance, but also to learn what kind of horror actually works on her. And it turns out, she appreciates it the same way I do. Case in point — the original Alien from 1979. All the atmospheric stuff worked like gangbusters for her: creeping through the spooky derelict, the tension of searching for the creature without knowing what or where it was… in these moments, she was very anxious, and enjoyed those scenes thoroughly. But the big set-piece chestburster scene? When the baby alien critter popped out, she laughed. She said it looked like an angry sausage.
That’s a perfect example of the contrasting reactions to different kinds of horror — not showing was frightening for her, but showing deflated the tension. But for another viewer, the reaction might be reversed; the setup is only setup and wouldn’t be effective without the payoff.
Consequently, in my view, the OP’s question is foundationally flawed, but it’s an understandable error because it’s not a flaw one will recognize unless one is deeply familiar with the genre.