Trump vs. Clinton vs. Sanders
What is the best way to decide whom to vote for? The President doesn't have a free hand in making policy. In economic and social policy, he must negotiate with Congress. However, in foreign policy the President can do pretty much as he pleases. In particular, he can take the country to war, as Bush did in Iraq and Obama did in Libya.
This gives us a test. Hilary Clinton voted for the war in Iraq, she supported the attack on Libya, and she was a strong supporter of an attack on Syria. Donald Trump opposed the Iraq war and considers the intervention in Libya to have been a mistake. He also opposes efforts to remove Bashar Assad as leader of Syria. Bernie Sanders opposed the Iraq war, thinks the intervention in Libya was a mistake, and doesn't want to attack Syria. His positions are very similar to Donald Trump's. The establishment Republicans have been much more interventionist. Marco Rubio advocated military intervention against Assad.
This means Trump and Sanders are acceptable candidates, while Clinton and most other Republicans aren't. I would vote for Trump over Clinton and Sanders over Rubio.