Sport's nutrition: what's the best macro ratio? carbs?protein?fat? for the purpose of building or maintain muscle mass (adding resistance training).

I think it depends: My brief research into this suggests it is debatable & depends somewhat on your goals: bodybuilding vs. fat loss vs. maintenance. More carbs are needed for body building, and many fewer for fat loss. 40-60% carbs are recommended for body building, with 25-35% protein and 15-25% fat. You might target somewhere in the middle: 50% carbs, 30% protein, 20% fat. Good luck!