Well, since I mentioned my love already for Go Nagai, I want to state my love for another manga artist whose a favorite of mine......the grandmaster of the horror manga genre, Kazuo Umezu.
Now, it should be pointed out that although the majority of Umezu's work is in the macabre/horror genre, he's actually done a good deal of work in other genres. The manga 14, which is considered to be his magnum opus, is actually a science fiction manga that occasionally has references to his other horror works. His early manga The Magical Medicine for Romance actually pioneered the romantic comedy genre of manga.
And of course, his most financially successful creation is his eternally popular gag manga, Makoto-chan. This gag comic came out around the time when there was a boom of vulgar gag manga. It's equally popular counterpart, Go Nagai's Harenchi Gakuen, was actually considered so vulgar that it was forced to end early due to controversy. However, whereas HG focused on sex jokes and nudity, Makoto-chan focused more on bodily functions, toilet humor and odd, surreal gags (including the famous "Gwashi!" hand gesture). This is best represented by the eponymous main character, who always bears a comically exaggerated runny nose.
But of course, Umezu is the god of horror manga, and it's there where he truly shines. Umezu's art style is well suited for this genre, due to the fact that it's simplicity gives the imagery an incredibly disturbing angle.....and the fact that children are often the 'victims' in his ghoulish tales. Furthermore, he's an incredibly talented artist who knows when and where to add extensive details in the right situations. Though his human characters are often drawn simplistically (the exception being the grusome deaths and injuries), his landscapes and monster drawings (especially youkai and ghosts) are striking, even nightmarish.
Of course, we know Umezu here in the States for his award-winning Drifting Classrom, though I personally think Viz screwed up the publishing. The original is six volumes, but apparently Viz decided to stretch it out to twice that.
In either case, it's a wonderful piece of work, and I hope that maybe if it's successful enough (and I'm really hoping here), maybe Viz will think about releasing it's sequeal, the aforementioned 14. Of course, that might just be a pipedream, but I can dream nonetheless.