Probably Cesaro. If Brock really is winning the WWE World Heavyweight Championship at Summerslam, then Cesaro holding the Intercontinental Title simultaneously will be a way for Heyman to brag that his two clients currently possess the two most prestigious championships in the company. And what a cool image it would be to see Cesaro and Lesnar in the ring at the same time, both with a title over their shoulder (in Lesnar's case, two titles.)
I know some are opposed to him becoming the IC champion since it seems to be a curse for some people, but the MITB briefcase that we all assumed was being kept warm for him ended up in the hands of Rollins instead and since Cesaro has nothing else of importance going on, you might as well crown him the new Intercontinental Champion. Besides, winning two prestigious battle royals will allow he and Heyman to boost about his dominance and success in battle royals when it comes time for the 2015 Royal Rumble match (who knows, maybe he'll be the last guy, or at least one of the last guys, that Reigns tosses out to win next year.)