I think QBs were ranked by things like total yards and yards per pass before the rankings were set up. That actually made some sense – it’s similar to the way baseball ranks batters (there are leaders in different offensive categories, but no one tries to put it all together for one number that defines the best hitter).
Thus looking at the 1968 AFL, John Hadl was probably considered the top QB, since he led in yardage, completions, and TDs.
If Major League Baseball issued a statistic that graded each player with a specific number, it would be picked apart within seconds and no one would pay any heed to it. But the NFL issued the passer rating and the media basically lapped it up.