Tom Brady is undoubtedly skilled, but he won the lottery of being drafted by Belichik and having the starter go down injured. Max Hall had "it" and would have excelled in an offense Belichik designed for him. Alex Smith had the tools to be a star with the right coaching. I think a few quarterbacks every year could have been as successful as Brady if they had twenty years with Belichik. A Manning, Luck, or Rodgers would be unstoppable.
I have a hard time believing that Belichik isn't the main reason for Brady's success when Bill can make stars out of several players from other teams' scrap piles and have twenty years of success dealing with the salary cap.