I'm (mostly) just going to discuss the more active artists.
As you've mentioned it really depends on style being looked for.
If you're looking for tribal, then BenD is pretty much unparalleled there. Branching off into other styles these days. There's others who do tribal too, but too many for me to care about.
Nightmare is great at realistic and the like. Armor, monsters, etc. Although you'd have to deal with his high minimum price and laziness.
You could also look for MANtrain to do a realistic set, but I'm not sure if he does requests and such these days.
If you're looking for robotic/armor I can say (with no ego, really) that I'm one of the best in that style. I'm also more than proficient at some other styles (i.e. gman's current set or Shin-Ryuu's). Alternatively, LANPENNER, Danvari and g0lg0 (among others) have some pretty good robotic sets.
And of course there's others, but I'm too busy to remember.