Ooo! Ooooo! That gave me week-long nightmares when I was much younger! So did the 1980's version of 'The Blob', 'Invasion of the Body Snatchers' (original and 70's), and 'The Island of Terror'.
I guess for me, what horrifies me are formless or blob-like monstrosities that can, make you suffer or hijack full control of your body, and possibly still being fully aware of what's happening to you and feeling everything, all while there's nothing you can do about it.
Mm, I feel that a little bit...
It's not that I'm 'unshakeable' or anything-- there are plenty of things that make me uncomfortable, like gore and other types of visual horror in particular. But I don't think being nauseous is quite the same thing as being scared. ^^;
If we define 'scare' as 'makes your heart race in a negative way, elevates your concern for the characters involved' I think the only thing that comes close to getting that reaction out of me is claustrophobic situations.
Like, if MC is hiding from the monster in a closet, it's whatever; I don't care, I might even still be laughing at that point (yes, I'm that kind of horror movie watcher).
But if MC is hiding from the monster in, like, a coffin, and you've got the camera jammed in there with them, NOW you have my undivided attention. 8[ I do not deal well with that kind of thing...that one scene from Kill Bill messed me up for years, and that wasn't even a horror movie...
There hasn't been a horror movie that scared me per se... as many, they feel so goofy. However, as a kid, the T-1000 from Terminator 2 was scary, as hell.
He's, basically, indestructible.. he can shape-shift and turn into other people and imitate their voices and he's a killer machine. The way he murders people is quite brutal and unforgiving. He's after a kid and would do anything to get him. Imagine yourself chased by something like him.
Forget the Exorcist girl, the T-1000 would slice her up and comes after you.
Jump-scares aren't really considered scary, it's just a Surprise "Gotcha Bitch!" moment. I am fond of steady types of horror where you can see the scary scene visibly and it is getting closer, and closer to the characters without jump-scares. That is the kind of horror I miss.
What I am scared of some scary women whose face is cover with her black her and it is crawling towards you like the grudge, or Mama. That gives me chills all over me.
The scariest things for me are bizzare / without explanation + a big part of it stays mysterious,
everything else maybe frighens me for a second
Horror things don't scare me as much as they should. Even my nightmares seem like pleasant dreams to me. Maybe it's because I know what's going to happen, for example, a group of friends hanging out in the woods with a serial killer on the loose or a supposed legend of a monster. Yeah, I know what's going to happen to that one guy that decides to wander alone in the dark. I enjoy horror because I like exploring the unknown, and I think monsters are cool and interesting, but scary? Not really.
I don't really get too scared of fictional works as much, but yeah I agree with you. I do get scared of chase scenes in horror games and if I take my headphones off I can play it without being scared, but that also kind of beats the purpose of the experience. I think what's the most chilling experience I have with video games that left me not wanting to play another horror game again is Dark Deception, especially in chapter 2 where I hear the monster breathing down my neck. Talking about it just makes me wanna run to bed and stay cover under the blanket for a while.
Something realistic that could happen to anyone at any moment scares me the most, like "Final destination", especially if it's graphic
I think movies like "Aniara" are what's truly frightening. That's no monster. The horror isn't "other people are monsters" (although the people are horrid to each other in the movie). The horror is the infinite vastness of space and how small we truly are compared to it. (If you know, you know!)
Something that inspires dread and turn your own mind against you. Just plopped into a situation where it seems like something is going to happen, and you're waiting for that something to happen even though it never did–the paranoia slowly driving you to insanity. That terrifies me.
And I did watch Hereditary once. It shook me to my core. Never again will I watch that movie. 
Wouldn't say it scares me, but body horror disturbs me. But not all body horror, mostly the kind of body horror, where someone's being turned into something else against their own will by any means. Bonus points to being disturbed if the process cannot be reversed.
I can't help but feel sorry for such characters. Especially if the end result is something like in "I have no mouth and I must scream" or "Made in the Abyss". Like even for the villain it's a bit too much of a punishment... xD
Also I guess it's more related to games than movies, but there's this specific type of jumpscare that just keeps on going. 99,99% of time it's just overdone regular jumpscare, where someone thinks loudness=scariness, but I've stumbled upon that 00,01% that actually scared me. Not because it was loud and "omg hyperrealistic blood and guts!!!!!1111", but like... Weird and cryptic. xD
@simonitropunk I love the T1000, Robert Patrick does an amazing job in that movie. I think that touches on a great group of frightening antagonists, too. Terminator, The Thing, the monster from It Follows...
They don't stop, and they cannot be reasoned with. Their thinking process may even be completely alien and incomprehensible... but unlike a simple natural disaster or animal, it's clear that they do have a goal: you. There's something uniquely horrific and personal about that.
"A survivor... unclouded by conscience, remorse, or delusions of morality." Exactly what you don't want chasing you around in the dark...
I think the best horror is ones that even just slightly bend expectations of reality. Often though, I find the more relatable the horror the better. Akin to @NickRowler paranormal creatures inherently isn't scary for me, though if executed well it can be great. I would possibly say the only exception to that would be the game anatomy maybe because of the unbearable expectation that there has to be some physical entity behind the lingering uneasiness.
That being said, I think the scariest pieces of media I can think of are the movies 'Midsommar' and 'District 9' not necessarily because of themselves but their created hyper-realities. they both arguably are effective and turning the audience into the victim and oppressor respectively.
Mild spoilers:
For Midsommar the scary part is how prevalent the idea was that the movie had a happy ending displaying how easily audiences bought into the cultish behavior that had brainwashed the protagonist clearly displaying the ease in which our minds are influenced.
The horror of District-9 comes often on a second viewing particularly in the popcorn scene of burning the aliens' houses. Most everyone I know (including) myself who have seen the movie finds the scene extremely uncomfortable upon rewatch because of how much they're reminded how easily they took part in the (speciest?) racist trap in the beginning.
Then there is the story the double by Fyodor Dostoevsky. A story I saw too much of myself in I swear it's convinced me that one day I will be replaced by a doppelganger that I see in my waking dreams. (A fear that actually inspired the first short of my series on Tapas)
I know it's not really conventional horror, but I think there is something to this kind of simulacra horror.
I've watched quite a bit of horror movies in college so little really takes me for a spin anymore, but for some reason Tusk really creeps me out more than most infamously "disturbing" movies. I legit got disturbed at the trailer even after watching other comparatively "worse" horror movies- just the body horror, and the horror of him even losing the ability to speak really gets to me.
I usually do prefer paranormal/supernatural stuff though, Tusk is the exception as usually I don't really feel that much fear for non-supernatural threats . . . plus, when it's more realistic sometimes it just makes me very sad LOL.
