I have a hard time picking between Babe Ruth & Jackie Robinson. The popularity of the game took off once fans watched Babe Ruth hit HRs. Also, Babe had a touch with fans, particularly kids. Without Babe, there may not have been the $$$ in baseball to reach so many fans.
Jackie not only opened the game up to minorities, but that event increased the skill level of the population of ballplayers as a whole. I wonder if Babe would have been so productive facing pitchers of the 1950's - or later.