What is the best form of meditation/yoga for a bodybuilder/boxer?

Many forms. I believe at least one "mind body" session per week will enhance anyone's training. There are different forms of yoga and i would just experience a beginner session to start then go from their. This will make your muscles work differently. You will improve your breathing and balance as well. This will enhance your body building activity. Good luck!
Abdominal breathing. Studies have shown that meditation can have positive effects on your autonomic nervous system, your immune system and your hormonal systems, thus rebalancing your overall metabolism. Proper abdominal breathing techniques are paramount in achieving the relaxation response. You do not need to be a master. Find someone who can teach you this and then practice it consistently.