Mickelson seems to be the most popular pick among the "experts." I can't say that I disagree, given his track record (6-time U.S Open runner-up). More importantly, it sounds like he's excited about the course, while a lot of others are already complaining, making excuses, etc. That's half the battle, in my opinion (not letting a course like this one get the better of your mentally).
I'd probably go Spieth, McIlroy, Mickelson, and pick your poison for #4.... Matsuyama has the best overall stats, in terms of driving, scrambling, and iron play. Rose has won big tournaments, but not necessarily on this type of course. Same with Fowler (and he's come close on links courses as well).
Guys like DJ and Watson surely have a good chance as well, but something tells me that this will be more about creativity and mastering the short game, than it will be about bombing drives. That (short game) is not necessarily the strength of either of those guys, although it sounds like Bubba has spent a LOT of time at the course, preparing for this week.