As mentioned, Whey Isolate is the standard protein to go, great amino acid profile. If you want to go even deeper, Research now shows that a blend of proteins with different digestive rates seem to promote muscle protein synthesis for longer than whey alone. Since protein synthesis leads to muscle growth, maintaining it for longer can have a significant effect on your results. Something like Whey Isolate, Egg albumen and Micellar Casein would be great.