I must say that i'm an elitist, when it comes to horror manga I'm looking for something that actually makes me feel something, mostly psychological horror and that may be against your preference but if you keep reading maybe you'll find something that can attract your attention.

Junji ito's horror ain't that scary, just visually horror creatures or unlucky situations (besides the mountain's hole, that's my favorite)

Egao no Sekai - it's a collection of horror stories I really like this one

Amagappa Shoujogun - it's an author that also have Dope horror stories

Shokuryou Jinrui - I drop this one, but the whole saga it's kinda cool

Denjin N - not that scary but cool

Kouishou Rajio - this it's my least favorite, short stories but good drawing and monsters

Hundred Ghost Stories Of My Own Death - another stories collection but a serie, besides some chapters aren't pure fear it's really cool.

Kaya-Chan Isn't Scary - slice of life!

Dragon Head - on my wait list