If he wins, he should choose Rubio as VP. Why? Because Rubio has the best foreign policy and national security experience, he is young but not too young, and because he seems to have a grip on diplomacy. He will need to smooth things over with congress when Trump boot stomps them every time he wants something done. And then Rubio will be ready to take over when Trump gets bored after one term and decides to opt out for a second term.

Just an off the cuff answer, because there are thousands of competent and viable choices to choose from.