All time? Stan Lee/Jack Kirby. There is no possible way that you can beat that. Kirby alone is probably the best writer/artist ever, and teamed with Stan "The Man" you get the Marvel Universe.
Currently, I'd have to go with Brubaker and Epting for their work on Captain America. While there are writers (Slott, Bendis, Millar, Loeb) and artists (Hitch, Mcguiness) I like more, these two are perfect together.
Worst? Pat Lee (nothing against his art but the book he was on was so off his proper subject matter) and whoever wrote the Punisher when he was some avenging angel..Never read it, but it makes me shudder.
Worst currently? I don't read any comics that I have the inkling that they might be bad, so I can't properly gauge what would be the worst out right now. I'll leave that up to the people that are going to spam this topic with "Bendis and Yu suck!" or "Bendis and Cho suck!" while they conveniently leave out that they buy both books monthly.