I was just thinking about this today. Is Manning the best QB ever to play the game? I think a strong case could be made that he is. With Manning the Colts are Playoff bound and possible Superbowl contenders. Without Manning they are getting blown out by a slightly above average team. With Manning, Reggie Wayne and Austin Collie put up Pro Bowl worthy numbers. Without him they are exposed as decent, but unremarkable wide receivers. Manning is 100% responsible for the success of the offenses he runs. Brady (while a great QB) and others benefit from working with excellent coaching staffs.
Manning may be the best I've ever seen. Take Tom Brady off the Patriots and they win 9-10 games. Take Manning off the Colts and you're looking at drafting Andrew Luck.