The best trainers: know training and nutrition theory and know how to teach it know training and nutrition theory and train regularly themselves train regularly, eat clean and know how to teach this to others know training/nutrition theory, know how to teach it to others, and train themselves

I believe the answer is: Know training and nutrition theory and know how to teach it

Best trainers in sport do not necessarily doing the training themselves, but they possess a deep knowledge regarding the training and had the communication skills necessary to teach them to the players. (which is why you can see that a lot of trainers in NBA, NFL, or even boxing are really out of shape)