The leanest protein source you can get is egg whites. Egg whites are the only protein source that have zero fat per serving.
Game meat such as horse, bison, elk, deer etc. is generally much lower in fat and cholesterol then beef, and also much higher in protein and nutrients.
Truth be told, commercial beef really isn't that great for you. It's been selectively bread to be very high in fat because that's what most people want - a juicy fat steak. Us as bodybuilders want the opposite. A lean and high protein cut of meat.