A question that takes some qualification. Football historians have usually rated quarterbacks apart from other players. They have always preferred running backs as "the greatest player in the game" to quarterbacks. Several years ago Jerry Rice was christened the Greatest Football Player of All Time. Before that Jim Brown was generally given that title. Rice got the title in large part due to longevity since Brown had "only" nine years. Pass receivers had never been considered "the greatest" before, Don Hutson came closest.
Tom Brady has played the most important position better than anybody else longer than anybody else. You could argue Brady was a better QB than Rice was a WR for an even longer period of time. Since Tom Brady is playing at top level at age 44, I would say he is now the Greatest Football Player of All Time.
This is my own historical perspective.