Panic breaking on a single road bike will result in the fastest stop if only the front brake is used.
On steep descents with switchbacks, it's usually best to use the front and rear brakes alternately, giving the other pad and rim a chance to cool, braking hard and then letting the bike run until the next turn. When I used to ride fast motorcycles, I'd set up a turn with light rear braking to get directional stability, then hit the front brake before diving into the turn.
The thing to avoid is locking up either wheel or hard braking during a turn.