Hiroaki Samura, author of both art and plot of Blade of the Immortal. There are others, but I think Samura is the most talented of them all: good plot, great, detailed art. Few but strong and beautiful female characters. Don't know if he's my favourite, but he's the best.
I also like Mihona Fujii a lot, too
I love the cleanliness and detail of her art, and she can get really funny.
Urgh! I forgot him! The great! The God! Osamu Tezuka.
I know Tezuka has a very simple drawing, but when you read his adult mangas you just forget. His manga for children I don't like, but the adult stuff such as Buddha, Ayako and others is great. The best thing he's done is his stuff about Japanese history. I don't like his futuristic series, but his series about Japan (the realistic ones) are great.