Some necromancy here, but I don't mind. And indeed, I'm shocked at no mention of Miura sensei as well.
I have too many to mention, but Kubo Tite has always been an inspiration in making cool looking characters and fight scenes. He just has a certain flow that can't be underestimated.
Hiroya Oku of Gantz because of the technical excellence of Gantz, I just can't fathom that many 3d models and that attention to detail. There are other impressive seinen artists who do this, but it's Gantz that really blows my mind.
Of course, in terms of sheer drawing ability Kentaro is a beast as well.
A random favourite of mine would be Sayman Dowman of Vovnich Hotel. Such a cute simplistic style mixed with some grim stories.
And how I wish I could draw like FLCL's Hajime Ueda
Geh, with every name I mention a new name comes to mind...