That 84 season of Marino's was by far and away the greatest single season performance by any player at any position in football. People rarely are smart enough to rank it 1st where it belongs.
I dont know Marino is my favorite player but rushing for over 2000 yards in 14 games I believe is on at least equal footing. Simpson averaged 143 yards a game for a season running and no one else has come within 10 yards of that on average. Both achievements are all time greats.