Typically a good blend of all essential amino acids in free form may be the best.
There are 11 amino acids considered essential (your body cannot make these so needs them from food) and from these essential your body will be able to produce all others (like glutamine).
It's also important to have good ratios between the EAAs as to maintain balance.
I particularly use Dr Vickery's Platinum Plus essential amino acids with good sucess.
I do take larger doses than recommended (if a normal person eats say 30grams of protein a day, that's about 30grams of aminoacids if digestion is good).
However the free form EAAs are to help the body make its own enzymes (amino acids are needed for this) and to strengthen digestion in general so that it can digest the protein (from food) and get more AAs.