Raw soup bones from the grocery store. I have the "butcher" cut them into 1 1/2 inch lengths so they are too large to crack open and small enough for the dog to get all the marrow out. After they have been licked and chewed up and are smelling "funky", I boil them which sterilizes them. They can then be kept "forever".
Mine also lone the hard Nylabones, preferably the dinosaur shaped ones. The poor things get decapitated rather quickly though. :p
Too funny! These two things, soup bones and dino bones, are the only things that my two B's LOVE! I would also add the Galileo bone made by Nylabones is super sturdy too. 🙂