I try not to give speed commands once an aircraft is on final. Separation should have been maintained beforehand. However, I assume best forward speed is the faster speed that your aircraft can safely fly without causing the pilot concern…flaps or no flaps. Basically do not fly faster than you can Safely land or do not violate the speed limitations.
The speed that helps in abridging a big gap between two aircrafts/to prevent an imminent collision- is best forward speed. It is best as long as you can slow down for a safe approach.