I've just started reading Zombie Powder too, and I'm already sold on it. I'm looking forward to the next volume for it.
Favorite manga-kas, eh? Hmmm....
Well, I've gotta say Naoki Urasawa. I've only read four volumes of Monster, so maybe it's a tad premature... but those four volumes have been so gripping that it's easily the manga I anticipate the most.
Next, I guess would be the Riichiro Inagaki - Yusuke Murata duo. Eyeshield 21 is pure sports comedy gold.
For shoujo, Kyoko Hikawa wins. From Far Away is my favorite shoujo-fantasy manga ever, with strong characters, good fantasy elements, angst, and bishonen.
Fuyumi Soryo also needs to go in here. Mars was enjoyable, but she's really hooked me with ES, which from what I've read is a promising series.
Matsuri Akino definitely has to go in here, for her manga Petshop of Horrors and Genju no Seiza.
And Hiroaki Samura too! Who can't help but love Blade of the Immortal!