There are a huge number of training systems out there, and many of them can work quite well - at least for a while. But if there's one single method that works better than any other for building the maximum amount of muscle in the minimum amount of time it's a full body workout routine.
Although body part splits are extremely popular these days, the main reason for this is because they are what all the top bodybuilders use. But top bodybuilders are not like the rest of us. For a start they are taking a lot of steroids to enhance their muscle gains. On top of that they are very genetically gifted. And also they are very near their genetic limits in terms of the amount of muscle they can put on. So they need a huge amount of volume and intensity to stimulate additional growth. And this can only be accomplished by using body part splits.
But if you are genetically fairly average, and just want to add 20 - 30 pounds of muscle to your frame as quickly as possible, the best way to do this is with a full body workout.
However you need to know how to do it right to get the best results...
How To Structure The Ideal Full Body Workout
A full body workout works so well because it allows you to train all your major muscle groups more often. And all else being equal, a greater frequency of training leads to more muscle growth. That's provided you can recover from it of course.
Another reason it works so well is that because you need to work your entire body in a short period of time you are forced to use compound exercises in order to get the job done. Compound exercises work the most muscle, and they also produce the highest levels of hormonal response. And this is what causes you to gain more muscle.
But you can't do exactly the same full body workout every time you go to the gym or you will soon get overtrained. So you need to build in some variation over the course of the training week. You also need to keep your workouts short; one of the biggest mistakes most people make is doing too many exercises. Three or four exercises per workout are all you need. Doing more will only over tax your system and make it more difficult for you to recover.
A basic full body workout simply needs to consist of a squat or a deadlift variation together with an upper body push and an upper body pull. Do this three times per week and you will get all the growth stimulus you need for maximum results.
So if you are a beginner you might just choose two different workouts and alternate these over your three weekly visits to the gym. Then, when you've been training for a while you could do three different workouts, so you only perform each movement once per week, but each major muscle group is still being trained three times per week. This might look something like the following...
An Example Of A Full Body Workout Routine
Incline Bench Press
Close Grip Pulldown
By alternating your exercises over the training week in this way you'll be able to recover better, which will mean better gains over the long term.
How Many Sets And Reps Should You Do?
Another factor to take into account is your training volume - the number of sets and reps you do. In general most people will find they will build muscle fastest when working in the 5 - 8 rep range. This will build solid, hard, dense muscle, together with a good level of strength.
However it's worth going up to the 10 - 12 rep range occasionally in order to stimulate more growth through sarcoplasmic hypertrophy. That is growth of the fluid components of the muscle cells, rather than the muscle fibres themselves. And by going down to around 3 reps occasionally you will be able to build additional strength, which will translate into more size gains when you are able to use more weight for the higher rep work.
So a good training volume would be to do 3 - 4 sets of 5 - 8 reps, after a couple of work-up sets to prepare yourself for the heavier work. After 4 - 8 weeks on a routine of this sort you could change this and do 4 - 6 sets of 3 - 5 reps for a similar time period. And then you could do 3 sets of 8 - 12 reps for a while.
It's also worth changing your exercises occasionally too, as this will prevent your body from getting too accustomed to them, and this will translate into more progress over time.
A full body workout routine really is the best way to build muscle in the fastest possible time. So give it a go in the way described here, and be prepared to make the best gains of your life.