Ronda Rousey is a great female athlete but I would hesitate to call her the greatest female athlete of all time. The reason I hesitate is because it is difficult to compare athletes across sports. Ronda Rousey is probably the greatest female fighter of all time.
She excelled at judo, mixed martial arts, and wrestling. She also had great success in ultimate fighting, which combines all types of fighting styles. Because she dominate ultimate fighting, I would have to say she is the greatest female fighter currently.