Rand Paul. He doesn't support the mindless 'the enemy of our enemy is our friend' foreign policy of the other Republicans, actually cares about the constitutionality of wars, is open to diplomacy, and understands that most foreign aid is a waste. No other candidate, not even Bernie Sanders, has all of those traits.
Any of the Democrats and Rand Paul are basically the only options for a realistic and non-nuclear holocaust foreign policy.
Hillary Clinton??
Hillary Clinton is obviously much farther to the right on foreign policy than I'd like her to be, but I'd place her just short of any non-Paul GOPer, and just short of nuclear holocaust.