I've tried my share of suppliments. The best one I've found so far is food.
Eat enough protein, eat the right amount of fat for your body and goals, eat enough carbs around your workout to get a good
insulin response and you will make the most anabolic hormone available to the
human body - insulin. Then you will grow.
Don't eat enough and you won't grow. Overdo it and you'll get fat.
You are a fitness trainer - you already know this stuff. Don't sell your clients on useless supps that won't do them any good.
Workouts - perfect
Nutrition - perfect
Then and only then - suppliments.
Suppliment list - creatine, food.
Maybe some GABA if they need a little edge once everything else is perfect.