Elway/Young/Aikman/Kelly are all pretty interchangable. Aikman won 3 Superbowls in 4 years, that's why I give him the edge. Yes, I acknowledge that Aikman also had the most talent around him of this group (with the exception of Steve Young's '94 team)... but honestly, I don't care what the team is around you, you get 4 shots at a championship and lose every one? No, you don't get ahead of a 3 time winner on this list.
Elway of the late '90s was not the same QB as Elway of the late '80s. John Elway was, for most of his career, Brett Favre with a lesser arm and an utter inability to read a zone defense. Elway ddin't join the "all time great" category until he started winning Superbowls.
Steve Young was the most physically talented of the four, by a long ways. He was more accurate than Aikman, more mobile than Elway, and as intelligent as Kelly. If it hadn't been for the loss of Ricky Watters... who they took 3 years to replace... I think he'd be the best among these names.