
What everyone else said. A tip I learned from chef, Tyler Florence on the Food Network, is that it is the slow heat AT THE END that makes meat (like ribs) fall off the bone, or stew meats so juicy & tender you can shred them.
As you are cooking, for a while, the muscle fiber seizes up & becomes tough. This happens especially if cooking too fast with high heat. When you continue cooking at a low heat beyond that point, WITH a liquid or sauce,) the muscle fiber breaks down, until it finally falls apart & off the bone. Slow cookers work on the same principle which is why it is hard to over-cook something in a slow cooker.
So if the ribs or any meat is still too tough, keep cooking at a
low heat!
People make the mistake of cooking at the end, with too high heat, not long enough, or not enough moisture. If your heat is low enough and you have enough moisture, you can almost never OVER-cook meat.
As for which meats are naturally more tender, they would be the more expensive cuts. The ones that are meant for broiling or quickly pan frying or stir-frying, like steaks & chops. When I want to make a
quick beef stroganoff, I will take a more expensive steak, cut it off the bone then slice it up into thin strips or chunks and toss it in so it just cooks all the way through and flavors the sauce. It stays very tender & flavorful that way. No over-cooking these kinds of meats as they lose flavor the more you cook them. They are meant for quick cooking.
Stay away from meats that say "stew" meats, or "braising" meats. Also stay away from flank steaks and the like that tend to stay stringy, even when cooked right, unless you know how to carefully cut them against the grain.
I also stay away from anything that says "chuck." They are often made into steaks or hamburger meant, but there is NO flavor to them.