Whey concentrate protein is the most complete, highest quality protein on the market today. When it’s grass-fed and combined with digestive enzymes, such as in Metabolic Fuel, even those with dairy sensitivity are likely in the clear.
If you must go with an alternative, pea protein is the second best option, but it is not a complete source of all essential amino acids. Hemp protein is a complete plant-based protein. Egg-white or beef protein are ok. Soy protein should never be used.
If you do not use Metabolic Fuel, make sure the protein powder matches the requirements indicated in the manual of the program as closely as possible.