I think it would be best not to characterize these macro nutrients as "best" for one purpose or another. Humans can live a healthy life with little or no carbohydrates in their diet, not so for fats and proteins. Fats are critical for brain functions, metabolic processes, immune systems, micronutrient uptake. Proteins are, as you point out, the building materials for our bodies. As I understand it we can metabolize either fat or protein to create the fuels our bodies burn, we can't metabolize carbs to fill the critical roles of fats and proteins.