I've learned that one of the best sources of protein is Whey Protein Isolate. It's easy to digest and packs about 17 - 20 grams of protein per one ounce scoop. I buy it in bulk and sweeten it with Stevia. Most store bought brands contain sugar or sucralose plus a lot of other additives.
Don't confuse this with Whey Protein Concentrate. Isolate is what you want. Here is a link to a page with some of the benefits:
Whey Protein Institute - Benefits of whey protein