The New York Times says it is Sir Alex Ferguson, recently retired coach of Manchester United Football Club. He won 13 league championships, five F.A. cups, and 2 Champions League titles. According to this chart, he also won 65% of his matches.
Here are some of the North American contenders:
Phil Jackson: 11 NBA titles and won roughly 68% of his games.
Gino Auriemma: 8 NCAA titles and won 86% of his games.
Pat Summitt: 8 NCAA titles and won 84% of her games.
John Wooden: 10 NCAA titles and won 80% of his games.
Red Auerbach: 9 NBA titles and won rougly 58% of his games.
Scotty Bowman: 9 Stanley Cup titles and won roughly 66% of his games.
Casey Stengel: Won the World Series 7 times with a 51 winning percentage.
Bear Bryant: Won 76% of his games along with 6 national championships.
Mike Krzyzewski: Won four national championships and roughly 76% of his games.
Paul Brown: Won roughly 67% of his games along with seven NFL titles.
Joe McCarthy: Won the World Series seven times and had a 62 winning percentage.
George Halas: Six NFL championships and a roughly 68 winning percentage.
Who would you choose as the greatest coach of all time?