Since we have a thread like this over in the anime section, I think we deserve one here as well. What are your favorite horror manga?
Ibitsu tops my list. It's terrifying in all the right was.
Bizenghast isn't half bad, with some genuinely scary moments and imagery.
Ghost Hunt since it mirrors the anime very closely.
Higurashi because it goes into so much more detail than the anime- in fact, important pieces of information that you don't get in the anime appear here. Wonderful!
Petshop of Horrors has a Twilight-Zone-esque quality to it that I really enjoy.
Dragon Head- While I dislike how it ended, it does semi-horror very well.
Ito Junji collection- Downright freaky most of the time, and that's a good thing when it comes to horror!