The most effective way to stop is by using your front brake and your front brake can provide maximum stopping power on it's own... this is really important during panic stops as you can cut your stopping distance in half.

To do this you need braking technique that has been developed to the point of being reflex as when you hammer the front brake you also need to shift your weight back (but this does vary by the rider and the bike) and know when to back off / modulate that braking as to not go otb.

Rear brakes are best suited for modulating speed and are especially important when the road surfaces are slippery as using, or mis-using a front brake under these conditions can be disastrous.