If you are looking for a post workout, fast digesting, get a good whey. Most people swear my Optimum Nutrition. I have a 5 pound jug of it and I hate it. It's clumpy when it mixes. GNC Wheybolic is good, but expensive. Out of all that I've tried, I prefer MGN (muscle gauge nutrition) it's cheap, tastes great and mixes well.
If you want a slow digesting, get a casein and drink it before bedtime.