Good question!
Five years ago I would have said Martin Kaymer... but sadly that's not going to happen now.
Mickelson isn't going to win another major, least of all the US Open. Spieth's game has gone AWOL, it's a long road back for him.
Rory only needs one more, so it's got to be him surely. Augusta is suited to his game, and he's played really well there, including last week. Of course there is mental scar tissue there, but he has been able to bounce back before. At some point in the next ten years the stars will align for him: he'll turn up on form, the course will be wet, he'll score well in round 1, get some good breaks, have a hot putter all week.
DJ should go on to win a PGA. He has the game to win an Open, he could well be favourite at RSG next year after he performance in 2011.
Koepka will probably win more US Opens and PGA Championships. I can see him winning a Masters but the only way I can see him winning an Open is at a calm St Andrews.