Start with fit, comfort and the requirements of your league. Flag football gear should allow players to move freely and perform without unnecessary bulk or restriction.
For gloves, look for grip, flexibility and a secure fit. For mouth guards and protective headgear, comfort and proper sizing are important. You should also consider the player's age, position, level of competition and how frequently they'll be playing.
Most importantly, check your league's equipment rules before purchasing new gear, since permitted and required equipment can differ between organizations.