tim bradstreet (his hellblazer and punisher covers rank among some of my favorite comics images ever) glenn fabry mike mignola david mack darick robertson
Dave McKean - everyone else is a distant second. Contemplated just repeating his name 5 times. JH Willams III & Mick Gray - Promethea Christopher Moeller - Lucifer David Mack - c'mon, now. Gianluca Pagliarani - assuming he did the painted retro covers of Ignition City
David Aja Dave Johnson Jock Jamie McKelvie David Mack
edit: Errr, I don't think any of these need any explanation, except maybe McKelvie. Those Phonogram covers were fantastic. Jock executes a single idea on the cover really, really well. Ditto for Dave Johnson. I mean, 100 Bullets, people. David Mack, obvious reasons. Aja, because, well, I really like his style, his use of blocks (see Immortal Iron Fist #16, Green Arrow #12) within the unifying theme, and he can do a "normal" cover and he can do the variants/wacky ideas really well. Aja's stuff is always striking to me, in short.
McKean. He's the only artist whose covers make me buy books I would otherwise not be interested in. I actually went and became an artist because of his covers. I had the same response to the question as razrangel, only I can't be arsed thinking of four more.
Dave Johnson - for Unknown Soldier Camilla d'Errico - for Elephant Men Richard Case - for Doom Patrol M.K. Perker - for Air and Cairo Jill Thompson - for Scary Godmother