Proteins are composed of amino acids, some of which your body can synthesize and others not. The nine you need yet cannot synthesize, and thus need to ingest, are called essential amino acids (EAAs). Among those, branched-chain amino acids (BCAAs) are crucial to your muscles, with leucine being especially anabolic.
|Essential Amino Acid (EAA)||mg/kg/day||Complete||Milk||Pea||Rice||Soy||Whey|
mg/kg/day = daily requirement in milligrams (of a given amino acid) per kilogram (of body weight) per day | Complete/Milk/Pea/Rice/Soy/Whey = milligrams of amino acid per gram of complete/milk/pea/rice/soy/whey protein (mg/g)
References: World Health Organization. Protein and Amino Acid Requirements in Human Nutrition, page 245, table 49. 2007. Kalman. Foods. 2014. Gorissen et al. Amino Acids. 2018. USDA Food Composition Databases (accessed: 2018 Sep)
A protein is called complete when, proportionally to its overall amino-acid content, it has enough of each EAA. The main advantage of animal proteins is that most are complete.