There is really not one that is the fastest and most productive. You can have a number of people following the same program but each one will react differently to that same program. I have clients who gained muscle mass by doing higher repetitions and less rest, while others gained when they followed the traditional heavier weights and less reps with more rest between sets. And don’t forget the diet part since this is the most important factor when it comes to muscle gain. I’m sure others might have different experiences with this subject and it will be interesting to hear them.