Tim Bradstreet Yuko Shimizu James Jean Fabio Moon/Gabriel Ba (listed together due to being awesome and working together lots) Rob Guillory (but largely for his art, I mostly think his drawing style is unbelievably awesome)
EDIT:
Stressing Yuko Shimizu since no one has mentioned her yet. Her covers for Unwritten are beautiful (and what got me to buy the trades).
Gabriel Rodriguez John Totlebon, or alternate universe John Totlebon who didn't get tragic eye problem if that makes it a more practical answer Michael William Kaluta Jacen Burrows Tony Harris
With some of my favourite covers linked (which has shown me a few interesting common elements between them, such as some characters making eye contact with the reader, which I seem to like):