You want one where a substantial amount of the fat is present in the muscle and evenly spread throughout. It's tasty and as the moisture evaporates out of the steak the melting fat soaks into the meat, keeping it tender and juicy.

Salt also softens meat, in part because of its effects on moisture evaporation temperature changing in the solution, but just as importantly because salt is what naturally forces muscle to relax in a living body as well as a cut of meat. That's why body builders eat bananas, they have potassium salts.