It'd make perfect sense for Ambrose to win the briefcase considering A.) Money In The Bank this year is in his home-state of Ohio, and B.) He was screwed out of it last year and was just screwed out of the WWE Title at Elimination Chamber last night as well, both times involving Rollins. Plus, Seth Rollins fearing for his championship all Summer long due to having Brock Lesnar chasing after him on one side and Ambrose waiting in the shadows for the right time to cash in on the other would make for some pretty entertaining television. But Dean and Seth are almost certainly gonna battle it out over the title again at the PPV, so scratch that one.
Without Ambrose in the match, Roman Reigns is the obvious answer... Especially if they want to book a triple-threat match between he, Rollins, and Lesnar for the title at Summerslam since the briefcase could be the catalyst for how Reigns grants himself another shot at the title.
Personally, I hope it's just like 2013 all over again, where it seems like it's headed one way but they decide to go with an unexpected winner instead.