Uzumaki: If you have read and liked Stephen King's Tommyknockers , you should definitely take a look at this horror manga. Like the small town of Haven in Stephen King's novel, the residents in Kurôzu-cho comes under a strange curse that lead many of them to do insane shit. The artwork is also not your common manga style and is really suitable for telling these kinds of stories. Very pretty too. 19 chapters with an additional bonus chapter.
Parasyte: Another 'horror' manga. This one can be best described as John Carpenter's The Thing meets Spiderman meets X-files. Though the plot is basically alien parasites invading human bodies and other living creatures, Parasyte is really a thought provoking series that discusses the food pyramid. The artwork here is plain but would not be considered ugly. Thanks to the crazy parasytes themselves and the violence, you will overlook the bland artwork. 64 Chapters.