Natuski Takaya - Fruits Basket (one of my first and favorite manga). Her artwork is really soft and gentle and draws me, and she's got a great sense of humor that shines in her manga.
Kubo Tite - Bleach, of course. He's got a really good sense of style. I love all the characters outfits, and the characters themselves!!
CLAMP - Awesome work. That's all I can say.
Kaori Yuki - I've only read Godchild, but I've seen her other works, and I just really love all her artwork, as it reminds me of The Spiderwick Chronicles or A Series of Unfortunate Events, for some reason.