The bigger problem, however, is this: It's hard, if not impossible, to compare QBs from different eras because they played under different rules, with different offensive schemes, and against different types of defenses. I think the best we can do is identify the best of each era, but even that is fraught with difficulties (e.g., there are many who think Manning is better than Brady but Brady had more success because he played for better coaches and teams). Still, it's far more doable than comparing Tom Brady with Sammy Baugh or Otto Graham.
P.S. As my friend, Tom Olson, points out, without Bart Starr the Lombardi Trophy would probably be called something else.