Would Mandy count as a horror movie? That movie is way the fuck out there. Either way it's goddamn good.
I think that anything Shudder hosts can be considered horror in one way or another, although they do have
Dave Made a Maze which I guess is kind of like a comedy slasher/stalker movie. I hadn't even thought of it but my friend informed me that the movies from The Last Drive-In are kind of time sensitive because they can only remain on Shudder as long as they hold the rights tot he movies that he's showing but it seems like they do a really good job of holding onto things, at least from the last season, for quite a while.
Shudder has been doing a great job grabbing a bunch of smaller movies and giving them some light.
Caveat ended up being a really good ghost story that I probably wouldn't have had access to without Shudder as well as The Witch in the Window.
An Unquiet Grave is a decent movie although it doesn't really seem like it should be feature length, the story could be pretty well handled in a shorter length in some kind of episodic series.
Hosts is probably one of my favorite movies up on their channel right now. It's British and pretty clever for what it's conveying but it does a really great job not going too far into gory but telling a story in the vein anyways. It gets pretty rough but still keeps its head and doesn't jump the shark into torture porn.
Porno is a pretty good throwback to shlocky horror of the 80's, which makes sense since it was produced by Fangoria.
The combo of
Resolution and
The Endless is a fun watch, they were made by the guy who made
Spring
An older one from the beginning days of Shudder but
Prevenge is awesome and stars and is directed by Alice Lowe from Garth Merenghi's Darkplace who has also done quite a bit of work with the guys from The Mighty Boosh and, I think, some Simon Pegg stuff.
Along the same lines as
Prevenge there's
Sightseers. Steve Oram, also from The Might Boosh crew (he's the homeless guy in the crackfox episode), has been in a few of my favorite horror movies over the past few years, and Alice Lowe is also in it. It more closely resembles dark comedy but it goes dark enough where it's horror.
Probably one of my favorites of all time at this point is
A Dark Song, also with Steve Oram
. It goes into Aleister Crowly black magick stuff and depicts a woman trying to perform the Abramelin ritual. It's pretty slow burn but it does an amazing job building atmosphere and tension
Sator was a decent low budget one. It's really low budget but the writer/director did a great job at telling his story. i think he used a bunch of recordings of either his mother or grandmother as they were aging and going a bit crazy and then built a movie around it.
I'd recommend
Channel Zero as well. My favorite was the Rutger Hauer season (3) but they all do a great job given that they are based on creepypasta and they sometimes had to flesh out stories that made up no more than a few lines. It's kind of a shame that it got cancelled because they were all incredibly well made for being on a syfy channel budget