Art Adams -- Longshot. Crazy detail that the coloring of the time just couldn't do justice to.
Moebius -- the graphic novels (Upon a Star, etc).
Paul Smith -- X-Men, I think the finale of the first Brood arc.
Bill Sienkiewicz -- New Mutants, demon bear story. Pretty much the pinnacle of newsprint/four-color comic art as far as I'm concerned.
Barry Windsor Smith -- Hmm, not sure. Epic Illustrated, maybe?
Gil Kane -- first issue of Conan I bought, the one with Pau-Styss or whatever that wizard's name was. I HATED Gil's art back then. Now I think it's awesome.
Michael Kaluta -- cover of Epic Illustrated; a Conan-esque pic that is still one of my all-time favorite fantasy pics
Thomas Canty -- cover of Mythago Wood.
Victor Ambrus -- An illustrated book called Tales of King Arthur. My friends and I referred to him as "Vic Superblood" because he did such awesome blood sprays (odd for a children's book). He also did some of the art in the Tolkien Bestiary (the elves).
Ian Miller -- Tolkien Bestiary (balrogs, dragons, trolls, I think the barrow wights, and a bunch of others).
Bob Pepper -- He did the art for the Dark Tower board game, and some really incredible art for a card game called Dragonmaster. Really unique style; almost has a stained-glass look to it.
Alan Lee -- Castles.
Mike Mignola -- Chronicles of Corum limited series. Mike is the king of style, simplicity, and design.