[Seven]

I'd argue Anson, Ruth and Jackie.

Anson was the first star to make the game marketable. He was the draw, he popularized the idea of a pitching rotation, and using signs as a means to communicate with players.

Ruth and Jackie because they ushered in new eras for baseball. It is ironic though that if it wasn't for Anson's extreme prejudice towards Black Americans, and the subsequent "gentlemen's agreement" that baseball should be segregated, then Jackie's impact would've been lessened, as their would've been no color barrier to break.