whey, or a whey blend that uses egg albumen or casein.
and there's multiple types of whey...whey isolates, microfiltered whey prtoein, whey concentrate, hydrolyzed whey peptides...
as you're in the US, I would start with a commercial brand like Optimum Nutrition 100% Whey. it tastes good and is a quality product.
soy isn't as biologically available as whey, and lots of people experience bloating with whey, especially men.
I'm not real fond of soy as a protein source in general. i don't run from foods with soy it them, but i don't seek them out either.