One also has to look at how much better they were than their peers. I posted it in the Gretzky thread. Jim Brown was so much better than anyone who played contemporary to him that he cannot be touched. Same with Jordan, Gretzky, and Ruth.
I don't know about QBs, Marino was great but he played in an era of great QBs. I don't think Brady or Mahomes are special from the standpoint of the position, but they won a lot of big games. When I think of a QB, I think big arm and an accurate passer- Like Jim Kelly or Elway. Peyton Manning was probably the smartest QB, Montana the best under pressure. But the best all time, I don't think there is one single QB that would fit that category.