I think the most important role on a baseball field is the pitcher because the pitcher basically dictates the entire game. He is the team leader and quite simply put it, he throws the ball and takes on the weight of the game while other teammates are there to support him. But they also rely on him to throw the ball to get people out and the pitcher is the person that coordinates the entire team effort and hence I would say that he is the most important. That's not saying that he can work alone and he himself makes the team, but he is the most important person on the field.