Of the two, my answer is Trump. It can't be Obama because he's ineligible to be President again. There is a maximum of two terms on that office, after all, and we had Obama for two terms. However, the is a chance that the Democrats will pick someone decent this go around and he/she could beat Trump. Trump's hope is that the Dems pick someone controversial or at least can't unite people, and to be fair they seem to have a knack for that as of late so it's a possibility. In addition, many Republicans aren't happy with Trump, so he may fail to reclaim the nomination. It's not so likely, but it's a thought.