there is no best hahah.. u buy one that fits your requirements.. there are many types of protein, but I assume ur talking about the usual one most bodybuilding people take which is whey.

if you are looking for the most affordable whey, our fellow lowyat bb forum-er 4rings sells it at a really good deal. search for him n pm him. if money isnt a problem, ON 100% whey is very popular and has been an all-time favourite. Scivation is also pretty good as it is one of the cheapest. So it really depends on your needs.