Black Simmentals, and I like to cross them with Angus. Good dispositions, good growth, fertility, longevity, marbling, most everything that's needed in range cattle. Those halfbloods are good cows too. I cross them back to Simmental.
That said, you must use the correct bulls to make any breeding program work. There are great bulls, and very bad bulls in every breed. I happen to have a very big advantage as an AI tech/salesperson in my ability to choose bulls that will work for me.