id say a varaity of lean animal sources provides a good combination of essential amino acids. I think varaity is the key to growth. whey proteins a good easy convienent source, egg whites turkey breast chicken breast steak,milk, cottage cheese etc.
I noticed that I started getting getting "harder" when I put refried beans into my diet with my chicken. I guess the beans were providing some missing amino that I wasn't getting. Therefore I would say get as many different types of protein as you can rather than pick one source as the best.