Trump for sure. Hillary Clinton is somewhat more of the same, have had presidents like her before. As someone from the UK, at least she would continue to hold to the Climate Change agreement. The US seems to be the only major power that has a likelihood of electing a head of state who denies climate change. That scares me because this is such an important time for action on the climate, and because without the US any agreements will be much weaker.

Trump also seems the last person you’d want in charge at a time when there is a lot of military instability in the world and concerns about nuclear weapons. He would be arrested probably here in the UK for inciting hate speech (the reason we had a petition to ban him from the UK). And to have an Islamophobe president at such a time as this seems a recipe for disaster.

Also, he has minimal understanding of foreign politics, and has made many gaffes. E.g. recently saying that Putin would not go into Ukraine, when he has already intervened there! Trump Insists Putin Isn't 'In Ukraine' And Would Never Invade It [Updated]

He doesn’t think before he acts, and after he acts, he just runs with what he said, and won’t back down but just digs deeper into whatever hole he has dug for himself (as with hs attack on a Muslim soldier's mother Ghazala Khan). That is surely not what the US wants from a president. He also seems unlikely to take the job seriously, but would just roll around like a loose cannon, reacting to situations rather than using diplomacy, and then digging the US deeper into trouble as the situation develops.

It’s a good thing the US constitution has some restraints on what a president can do or I’d be really scared.

As it is, well I don’t rate his chances high. He repeatedly shoots himself in the foot, shouts ‘Ouch’ and then blames his opponents for the harm he caused himself. I think that probably works better on the campaign trail than in an election, but if he does get in - surely he would not last more than one term once people see what the effect is of having him in power? I hope not. But 4 years of Trump as president would be bad enough.