I was thinking what was one of the most addicting, and well written manga I've read, and in popped into my mind Parasyte.
Summary
A teenage boy, Shinichi has a worm (parasyte) that enters his body and gets trapped in his right arm. "Because the Parasite infected his hand instead of his brain, Shinichi and Migi each retain their own, separate intellect. This gives them an edge in battling the other Parasites, who see Shinichi's ability to think independently as a threat. Shinichi also feels compelled to fight other Parasites, who kill and eat humans as a source of food." <--took from wikipedia, im lazy. Also it has an greatlove story and the ending is AMMMMAAAAZZZZING. One of the rare mangaS where i was satisfied after reading it. Starts out weird and very violent, but it got very addictive.
READ IT HERE: PARASYTE