I don't ready very much horror manga (maybe just because there aren't many of them in Italy), but I totally liked The Black Museum - Springald by Kazuhiro Fujita and Daydream (though this is a bit weird °°) ;D
I wouldn't consider most of things people mentioned as horror. But that's all about what you consider in a genre.
Nor are the two that I'm going to mention scary, but for a good monster manga, I'm with BioMeat: Necter, and Franken Fran. Both have a lot of classic elements of monster movies. Franken Fran is knowningly silly while having a very dark sense of humour(and it manages to be unsettling sometimes).