It is impossible to lump players, FROM DIFFERENT ERAS in to one bag and pick out the greatest of them all.
What makes one "the greatest?"
There are many factors involved.
There can be a "greatest" from a particular era in which all of the players in a particular category played under the same circumstances.
But to take the "greatest" from the current era, let's say 2000-2012, and compare it to a player from, lets say, 1960-1972, just cant be done.
Playing conditions were totally different, from the fields themselves, the equipment, the mindset, even in the NFL, the look, and feel, of the ball itself is not the same.
I might agree with the greatest players of the decade, but "ever", IMO just cant be done In all sports.
Even athletes that are not human, would Seabiscuit, for example, have been even greater had he run in an era where the condition of the tracks was better, feed was better, and jockeys wiser and smarter?
In that vein, is Brady the best ever? Maybe yes, maybe no. Until we can resurrect some of the players of the past and pit them against players that came after them, and let them go at it... we will never really know.
Would make for a pretty good movie though.