The most beneficial protein for muscle building is whey protein. It contains all essential amino acids and is rapidly absorbed by the body, making it ideal for post-workout recovery. Its high concentration of branched-chain amino acids (BCAAs), particularly leucine, is crucial for stimulating muscle protein synthesis. Additionally, whey protein supports muscle growth and repair, especially when consumed after resistance training. Other beneficial sources include casein, soy, and pea protein, though whey remains the gold standard for its overall effectiveness.