In normal riding conditions, (bike without linked brakes)
Front brake - High speed
Rear Brake - slow speed
When road surface conditions change obviously it's down to common sense and experience.
I learn't this at a track day, why does the front wheel have two discs and calipers and the rear only one? This obviously identifies the amount of stopping power available at each wheel.
When travelling at high speeds rear brake isn't going to stop you, too much and youv'e lost the back end.
Front brake however, with a bit of practice you can actually stop very quickly. The weight tranfers to the front compressing the suspension and pushes the tyre into the road surface giving loads of grip. Obviously this is in a straight line.
Braking during cornering should be avoided, this upsets the geometry of the bike, certainly with front brake. If you do need to lose a bit of speed in a corner, a liitle bit of rear brake is better.
At slow speeds if you use front brake it is too sensitive, the front end dives and steering goes heavy because you are pushing the front tyre into the tarmac. Using rear brake at slow speeds keeps the front end light and allows easier steering, this really hepls when manouvering at slow speed.