this question is freaking impossible. every artist has unique strengths and weaknesses. it's like acting who is the best painter. they all try to accomplish different things through painting, and the things they want to accomplish all require different strategies.
as far as pure skill goes, i'd probably say Ross. he knows the most about form, perspective, and all that jazz, and it is not like he lacks story-telling so much that his art gets boring and plastic (Land)
as far as style goes, i'd probably say Immonen. he's like a new Steranko. he gets lots of awesome things going on, and it just looks really really hip and stylish. i really think his nextWave covers are incredible, even though they all follow a general formula, as they continue to look awesome and different each time. i think he is the best graphic designer.
now in the middle lie artists which i think are the most enjoyable: Sook and McNiven come to mind. Both of them do really cool stuff with line-weight and detail and stuff (just came to mind: Immonen's lineweight is awesomly heavy), yet they have style. So as far as comic artists go, I think ones like them (probably McNiven) are the most suited for the job of making a generic comic, while others fall into more nichier roles (wouldn't want Immonen on Batman).