You can't compare across eras without adjusting for the league averages of each era.
I did exactly that a few years ago. I took 10 season increments, determined the average passer rating, subtracted that rating from the current average and added that to each passer rating for QB's of that time.
For example, between 1970 and 1979 the average passer rating of the top 20 QB's (given a minimum of 1000 attempts) was 74.34.Keep in mind that this was before "in the grasp", "helmet to helmet", and other "excessive force" rules. In other words, the play wasn't over until after the D-lineman had slammed the QB to the ground.
Anyway, the current average of the top 20 QB's was 93.87. This is a difference of 19.53 which is added to the passer ratings of the top 20 in the 70's.
Comparing QB's of the 70's to the present, the top 20 looks like this:
1 Aaron Rodgers
2 Roger Staubach*
3 Tom Brady
4 Drew Brees
5 Bob Griese*
6 Bert Jones
7 Ken Stabler*
8 Peyton Manning
9 Fran Tarkenton*
10 Russell Wilson
11 Ken Anderson
12 Tony Romo
13 Greg Landry
14 Ben Roethlisberger
15 Matt Ryan
16 Len Dawson*
17 Philip Rivers
18 Billy Kilmer
19 John Brodie
20 Kirk Cousins
Dak didn't qualify, he has thrown slightly less than 1000 passes. If he qualified, then he would have placed 13th, just behind Romo. Again, this was only a comparison of the 70's to the current decade.
When you add the other decades, I believe Staubach was 3rd best behind only Steve Young and Aaron Rodgers. Bart Starr was high up there from the 60's as was Joe Montana from the 80's. I'll have to redo this some day.