Wow, I'd thought originally to write to the 2 or 3 people who replied, but now it seems overwhelming. Thank you all for sharing!
Frazetta is "the master." When I was young, I didn't connect with his work, but I couldn't discount it; his work was always striking and powerful. As I got older, I've come to seriously appreciate his work.
Michael Whelan is "the face" of 80's and 90's scifi novels. I could look at his work all day. His work alone has sold me many paperbacks.
Larry Elmore is D&D to me. I can't say I love all of his work -- in fact there are a number of pieces I do not personally like -- but IMO his best work is incredible... especially that sense of motion. Yeah, I have to get some Elmore art! There was an ok documetary recently about the art and artists behind the early years of D&D... Eye of the Beholder?
I enjoy many of the artists who have made careers from Lord of the Rings (or used it as a springboard) but the Hildebrandt Bros are by far my favorite. Their storybook style is mesmerizing... and got me to buy the first Shannara books as well (they painted the covers).
I have a number of illustrators and comic artists whom I love. They could be on this list -- Kaluta, Vess, Gianni, Wrightson (who I was lucky enough to call a friend), etc -- but I'd be writing all day
And yes, having just recently looked into Warhammer, there is some amazing work (and artists) around those series... although I'm most partial toward Warhammer Fantasy artists.