It probably goes with the name of the genre, but the only thing that's essential to me in horror is that I feel both unsettled and grimly fascinated the whole time, so obviously I'll read any kind of horror so long as it's horrible. So I want every scene to have horror elements, even if it's just strange conversations or eerie walks, but I don't want to be terrified the whole time or it wears off. Someone else mentioned footsteps and warped reality - I like this as a 'foundation' to the horror scenes, but I'd need something really horrific to kick-off to be satisfied with the experience unless the horror is separating reality and delusion and the protag slowly descends into madness, that sort of thing.
But as a favourite: gore, gore, and more gore. I love body horror, and I love cosmic horror, and it's even better when those are linked. iia on nosleep wrote one of my favourite horror series with cosmic horror, shady governmental science programmes, body horror, and fungi. I'd rather be sickened than scared with horror.
I'm also pretty partial to when normal things turn ... not normal. So cults that might actually be eldritch, spooky things in the woods, and then I haven't read the book, only seen the film, but Jack Ketchum's The Girl Next Door was fucking horrifying just because of how normalised it felt. I also like to see strong motifs and occult images within it.
I don't really like ghosts because they don't give me the kind of lore I like and they lay too far outside 'normal' to be satisfying that way. I'm just not a big paranormal fan.
Also pls don't give me people making stupid choices. How much scarier is a story when the protagonists make all the right choices and it doesn't make a damn difference because there's nothing they can do to escape?
(pls forgive my essay that doesn't actually say anything)