Historically, it's gotta be Nick Cardy. Also, Carmine Infantino, when he was DC's Art Director. Also, Jack Kirby but I guess he's no longer an option. (*sniff*)
I like quite a few of Rob Liefeld's covers. His stuff is memorable, and Leifeld is going to have the last laugh in a few decades when his originals become hugely collectible because of the notoriety he gained during his career.
You can throw Michael Turner in there too. Like Liefeld he's completely shite at anatomy, but he has a sense of style and colour that's going to see his stuff becoming prized as collectibles in thirty or forty years' time.
@Greasemonkey: just like Liefeld, Michael Turner was a penciller, so you can't hold him responsible for the colors of his covers, only for the linework... The bigger comics publishers manage to churn a lot of comics with bad cover art that's made palatable by colorists!