You call any qb the greatest and there is a valid argument against it. Brady is as good as any that has ever stepped on the field. Whether he is the best is only an argument for fans. It can't be proved.
And as someone else already pointed out, if Brady was a Bill, the answers here would be significantly different.
Brady's career has been magnificent. The only argument against him is that he has played for the greatest coach I have ever seen, including, Lombardi. It is much harder for a team to remain good now than it was then. Those of you who cling to this notion that Belichick isn't an amazing talent because of the scandal are blissfully unaware of what every team does in order to win.
I remember at Giants games when opposing FG kickers were lining up, they would open the doors at one end of the stadium in order to change the winds to sabotage the kicker.
Then there's steroids which brought the Steelers tremendous success. Imagine all of the things we don't know about.