The best high protein food is packed with digestible protein that your cat's body is able to break down. Protein digestibility describes the proportion of protein in the digestive tract that is absorbed by the body. The more digestible the protein is, the more will be taken into the body where it can be made use of. This is why digestibility is so important.
Unfortunately, the digestibility of a protein is not indicated on cat food labels but you may be able to find it on the manufacturer's websites. As a rough guide, muscle meat is 92% digestible, organ meat is 90% digestible and fish is 75% digestible. On the other hand, corn is only 54% digestible for cats.