Hmmmm.....
Well Nightmare has some pretty detailed work and all, but to request something, you have to basically sell him you life. (Considering he doesn't make anymore.)
Hanz0 is the master of Robotics and bevel, makes his sets the most realistic and 3D-ish. His sets cost quite a lot as well, if you want a good one.
Bend's style is tribal fusion, tribal with a dash of his own style, something like an abstract anime? He can get you a pretty good deal, most affordable.
Then there's Mantrain who is probably one of the best there is. His anime heads simply can't be beat, and are probably the most life-like. I'm not quite sure...Doesn't really advertise that much, but is still quite win.
Then there are the 3D/Render people.
Stupinator- Can make some pretty good animation and renders.
Freelancer- Quite win at doodling fantastical crap inside his head.
Nightmare-Pretty much all around detailed and amazing at freedrawing, which he puts into all of his arts.
Then there are the less known, yet still good artists.
You have Novitech, who is quite well at making just about any avatar/signature, and has some pretty neat affects that make them special.
g0gl0, (Hope I spelt that right) is a undiscovered jewel, and makes his sets realistic and shiny.
LANPENNER again, quite undiscovered, yet has some quite amazing detail in his art.
There are some others, but I am in too much of a rush to name them.