The MLB manager who has won the most pennants in the American League is Casey Stengel with 10. He was managing the New York Yankees (1949-53, 1955-58, 1960).
The MLB manager who has won the most pennants in the National League is John McGraw with 10. He was managing the New York Giants (1904-05, 1911-13, 1917, 1921-24).