From all possible supplements I prefer glutamine, as it helps muscles to regenerate faster. I prefer it to pre and post workouts because it does not give you extra energy for workout, but you stay in gym as much as you can and still keep your muscles ready for next day workout.
From all the possible glutamine powder providers I prefer OPTIMUM NUTRITION, as all their other bodybuilding supplements, that’s a great brand, you may read a review at http://testozilla.com/supplements/best-glutamine
I agree that a balanced diet is key but people use supplements any way so it is better to be educated on them. Check out AdvoCare, they are independently tested for over 218 banned substances. I have a cert in Nutrition and have been using them for over 2 years with amazing results for my family, friends and clients. https://www.advocare.com/121021110/default.aspx