Best quarterback of all time and why?
I think that this is a more difficult question that may be readily apparent.
My vote goes to Joe Montana, despite hating the 49ers back when I was a kid.
I think it's especially difficult to compare quarterbacks from different eras for various reasons, and one HUGE reason in particular- remember these days you can barely even TOUCH a receiver as a defensive back, and this makes the passing game far easier in my opinion, which is why I would give Montana the nod over more recent future-Hall-of-Famers such as Brady and Manning.
Also, while he is definitely NOT the greatest QB of all time, I'll add that Jim Kelly is underrated. Sure he lost FOUR Super Bowls in a row, but to rack up all that passing yardage and all those wins when half his games are being played in cold and windy Buffalo is a heck of an achievement.