The best protein powder is a pure whey isolate. It is virtually void of all fat and calories. It is 90% protein by weight. A whey protein concentrate is about 80% protein by weight and will contain some fat and carbohydrates with each serving.
I personally use the whey isolate protein isolation from n2bm.com. It is a pure whey isolate that is naturally flavoured and sweetened with the non-caloric natural sweetener stevia.