I'm going to preface this by stating that I am pretty desensitized to horror/shocking content.
>>261357892Some manga in the survival genre are pretty good at delivering a harsh and unforgiving setting that lends itself well to horror. I've been reading Monkey Peak and it's been pretty solid from what I read, the tense moments are tense and the writer isn't shy about killing off characters.
Some manga that are closer to the drama/thriller genre are also excellent, like Blood On The Tracks. (I haven't read the ending yet, I stopped reading updates so I could read it when it was complete but never got around to that.)
In the horror genre itself you have a few gems that I've already seen mentioned in the thread, like Seeds of Anxiety and PTSD Radio. I've been following Hundred Ghost Stories Of My Own Death, it has a pretty effective framing story centering around a boy living with shitty parents too. I just wish the releases would come out faster.
>>261358502They are really good at body horror too.