What is the best meat source?

If you have to choose a single protein food, (limited ingredient diet), which of these do you think would provide the most nutritional value and be easiest on the digestive system: chicken, beef, fish, turkey, duck, venison, lamb, or rabbit?

Hmm, you may need to give a little more context?

If you are doing food trials - ie restricting the protein source to a single protein, and slowly adding new proteins over time to find out what makes them sick or just to condition their tummies to the new proteins, then I'd start with something that is generally not a known allergen, and something that is likely to be hunted and eaten in the wild.  To translate, cats wouldn't necessarily chase down and devour a cow.  They would be more likely to eat rabbits and birds.  And fish are one of the top 3 cat allergens- so i'd avoid that one.  If this is your situation, then I'm having the same situation.  We started with rabbit, as we knew that was loved...some people would start with poultry, but perhaps a novel type like duck if there were existing problems (seeing as chicken is in a lot of food, so it could be the cause).

If you are looking at just feeding one protein forever as an easy meal choice, this probably is not going to be good.  Variety will always be better at giving different nutritional imputs.