The term "best" is very subjective when talking protein or any other supplement for that matter.
When you say "best" are you referring to most protein per serving, calories per serving, sugar per serving, other ingredients in the protein, I could go on and on.
I will give you my recommendations based on what I know.
I have used ON (optimum nutrition) for years and have received, what I would call "decent" gains from it, and I say decent because I also take other supps along with the protein, and also ingest loads of proteins from my daily meals, therefore not REALLY knowing the full results from the protein powder itself directly.
I would recommend it as a good protein as the R&D behind it goes back for years and it has seemed to change with time, meaning they havent just derived a powder and stuck to it, they have revised their ingredients based on results.
I do not recommend Muscle Milk, although prolly the best tasting protein on the market. It carries, or used to I should say, I havent looked into it for quite a while, but it used to have loads of sugar and calories in it.
Isopure is another GREAT protein formula!
You really need to figure out what it is you want to gain from taking it and just research each product. It has taken me years to compile my archive of information pertaining to the said topics.
Two questions to ask yourself.
Are you looking to bulk up (and cut down fatty tissues later) or are you looking to build lean muscle mass, the answer to this will tell you a lot about which formula to take.