Hellsing
I love this manga so much ok. Kouta Hirano is amazing for creating this masterpiece. It literally has everything you could want in a dark manga. It has some humor here and there, the MC is OP but he's done right, the boss is a badass and isn't portrayed as weak just because she's human, the other main female MC gets some phenomenal character development (well actually a lot of characters do but hers stands out to me).
Tetragrammaton Labyrinth
How a yuri needs to be done. While there are some good school setting yuri stories, not a lot of them go for the more "actiony" side of things. This does it and does it well. All the characters get some kind of development and the story isn't perfect but will surely keep you interested.
Assassine
Wish Hirano would update this.
Drifters