My top 3 choices would be John Kasich, followed by Rand Paul, and lastly Jeb Bush.
This could possibly be the first time I vote for a Democrat,
If it ends up being Donald Trump vs Joe Biden, I'll be voting for Biden. I have no idea what would happen if Donald Trump becomes president and I DEFINITELY don't want to find out. Biden would probably follow the status quo and not a whole lot would change, which would suck, but the potential for the country to be severely messed up by a Trump presidency is to high to vote for him.
For me, the worst case scenario is it ending up being Donald Trump vs Bernie Sanders. If either one of them becomes president, there is a high probability of the country having at least one massive change that I probably won't like. Kudos to Sanders for getting all funding from normal people, but I don't want any more socialist entitlements or to have my high capacity guns taken away.