What is the best hydrolysed whey protein to buy?

I have used Vyo Pro by AST, Hydro Whey by Optimum, Iso Whey by Dymatize and a number of others. The main thing you're looking for is 100% hydrolysed whey isolate. Generally, you get what you pay for with these premium proteins. A lesser-priced hydrolysed whey is probably only partially hydrolysed (so much of the protein will be the same as found in normal whey protein).

So just look for a good brand and that it's 100%!