Most overrated: Neal Adams- all flash, no substance. Not great page layout and a lousy story teller. Visually dynamic, but not the genius he' d tell you he is.
                       Steranko- great design sense, but not a great artist when it comes to sequential story telling. Stunning cover artist, though.
                       Ditko- I'll hear cries of sacrilege over this, but I don't think he's a great artist. A genius when it comes to design and storytelling, though. 
Most underrated- John Severin- simple page layout and great storytelling. As a draftsman, he's an artists artist.