When looking for a a good protein powder, you need to know exactly what you want that protein powder to do for you. There is no such thing as the "best" protein powder available, because protein powders will vary in their effectiveness depending upon what you are using it for.
As a general rule of thumb (and I'm going to speak broadly here), it is best to go with a whey based protein powder for consumption prior to and immediately following a workout. A whey based protein powder high has the highest biological value (or BV), which effectively means that it is utilised by the body to a higher degree than any other protein available.
With whey protein powders, there are two main types. There is whey protein concentrate and whey protein isolate. Isolates are processed to a greater degree and consequently are absorbed into the body quicker than concentrates. Due to the additional processing, whey protein isolate (or WPI) is generally more expensive than whey protein concentrate (or WPC).
We have a few protein powders available which vary mainly on their WPI and WPC availability and thus price:
Dymatize Elite Whey - 2.27kg (more WPC)
Optimum Nutrition 100% Whey Gold Standard - 2.34Kg (a mix of WPI/WPC)
Dymatize ISO-100 - 2.27kg (pure WPI protein)
Of course there are many other proteins available on the market. Here's a couple of examples. Casein protein is absorbed over a seven hour period and thus could be useful for ingestion prior to sleeping. Soy based proteins, whilst posessing a low biological value, are ideal for many vegetarians.