I really liked
Knights of Sidonia, it's got some cosmic horror elements that I enjoy, as well as the lonely atmosphere that accompanies the last known bastion of humanity drifting aimlessly through space, fighting a desperate battle for survival against an enemy they hardly understand.
Kumika no Mikaku is also a nice read, but the sci-fi is very light and is more just a slice-of-life/food manga with a touch of romance.
Ogeha might be a little disturbing to some but if you aren't squeamish it's an interesting story of a possibly autistic middle-schooler finding the chrysalis of an alien creature in the woods, popping it prematurely and dragging her out, keeping the oddity as a pet and hiding her in a box within his bedroom.
One of my favorites and a widely known title is
Parasyte, where the main character's arm is consumed and replaced by an creature of unknown origin that originally intended to take the place of his head, and details their ensuing struggle to both avoid discovery by human and other parasites, as well as to protect his loved ones from the predatory monsters, which have a ravenous appetite for human flesh.