Example extended
Answer: In this case, the voltage is being held constant due to the nature of the batteries. This means that the current will change as we change the resistance. Thus, the P = V2 / R would be the most straight-forward equation to use. This means that as R goes down, P goes up.
The answer: for more power, lower the resistance. This will allow more current to flow at the same voltage, and hence allow more power!