Mattis. Say what you will about performance but he’s absolutely qualified. There were a handful of others like McMaster but nearly without exception they were quickly chased off after deeply embarrassing or compromising themselves.
Trump’s approach seems to be appoint a few at least vaguely qualified people to certain positions. Get them to dirty their hands. Then get mad at them for something, fire or chase them off. Replace with cronies. Seems to be an attempt to placate the rest of GOP leadership.