Mill Creek (Bogan Park), hands down. Usually have a number of Pony and Senior teams, and they play other county parks as well in a rec partnership league.
For the past few years, Norcross and Brookwood have had the top teams in Gwinnett. Typically 4-5 teams at 13/14 and also play inter-park schedule. In the spring leagues are larger and typically you will find Norcross and Brookwood playing in the finals of All Star tournaments. Not that this is the only metric but a lot of talent in these two parks. If looking closers in, NYO also has a great program, park is in Chastain area.
Be cautious at this age group, good coaching is a valuable commodity. By the time a player reaches this age group, the good coaches are with travel teams. Some parks have been so desperate that they place adults into coaching positions without any playing or coaching experience. A WARM BODY! Why is that wagon ahead the donkey? Just something to chew on folks! Check on the coaches, speak to them about their playing and coaching experience. Remember watch for your player first.