After your workout you want a protein that is quick digesting. Whey proteins are very quickly absorbed into the body, and have a high concentration of branched-chain amino acids, or BCAAs, which are highly concentrated in muscle tissue, and are used to fuel working muscles and stimulate protein synthesis.
Whey protein isolates is created by an extra process used to filter the proteins. Whey isolates yield a higher percentage of pure protein and can be filtered enough to be virtually lactose free, carbohydrate free, fat free, and cholesterol free. So they are the best thing for your post workout shake.
Have a serving of whey protein isolate after your workout.
(If you are sensitive to whey for some reason, egg protein is nearly as good.)