I think it all depends on how you define the best QB of all-time I would say
If you want championships Otto Graham has 7,
If you want Stats Dan Marino is your man,
If you want Innovation, Sammy Baugh invented the forward pass,
If you want a clutch player Joe Montana is your ideal pick,
If you want the ability to improvise you have to go with Fran Tarkenton,
If you want leadership Johnny Unitis is the guy,
If you want a Field General you will say Peyton Manning,
If you want a rocket arm and Likableness Brett Farve is your man,
And If you like some are in complete love with the modern era you will say Tom Brady,
My point is it is impossible to say who the best QB of all-time is because everyone has there own curriculum that they look for in a great QB, and that makes it impossible to say who's the best unless there is an established Curriculum made by the NFL for finding the best QB of all-time.
Stats are impossible to compare between players because of the fact that they all played in a different Era against very different player and different play calling tendencies, for example you can't compare Dan Marino and Sammy Baugh simply because Sammy got to pass the ball like 10 times a game before the roughing the passer and pass interference call were ever created, while Marino was protected by the rules alot more and got to throw the ball 30 times a game.
You can't use championships because guys like Marino played on bad teams most of there career
You can't use leadership ability because modern era QB have more cameras around to see there leadership ability then players like Sammy Baugh,
So please NFL, Hall of Fame, somebody make us a curriculum that can makes sense of all this and put this debate to an end once and for all.