It wouldn't surprise me at all if the winner is someone who hasn't even announced their candidacy yet. That's how bad the entire field is presently. Of the people running, Trump and Sanders would be the two most likely to be candidates. A lot of the result will come down to their running mates.
Someone like Tammy Duckworth would sway a ton of votes if she was on Sanders' ticket.
Biography
Trumps biggest problem is I don't think he will be able to attract a marquee running mate and that will weaken him in the general election.
In any case it's a terrible selection compared to years ago when you had good candidates on both sides like Bill Clinton vs Dole.