Let it go. This is an exercise in utter futility. There have been many and there will be more to come and what does it matter anyway?
My thought is you grade it by decade or era. What is an era. I would say 20 years is an era. So Montana gets the nod for 1980 through 2000. Brady is clearly the QB of the 2000 and 2020 for the time being. Manning is great, Brees and Rothliberger and Warner are also pretty damned good but trying to compare QB's from the 1960's,1970's to the Montana's, Young's, Elway's, Marino's etc is just silly. The game has changed.
Right now there are multiple top-notch QB's. Some might never be known or recognized as such but the caliber of athlete at the QB position is just out of this world. Schaub in Houston is pretty dag gone good and with A. Johnson to throw the ball to...whew! Aaron Rodgers is nice and G. Jennings as his main guy is something special. Brett Favre???? I kind of like him just because he is flying by the seat of his pants and you just feel like he is scratching plays out on the turf or not even following the plays that are called and he is just completely winging it. The interceptions are an issue, sure, but when he is on like he has been for the last three seasons it is not all that unrealistic to see him in 2020 as the MVP, ala the SUPER BOWL commercial.
Just enjoy the play and quit trying to rank everyone all the time. Is is nauseating to read some times.