Triple H vs. Rey Mysterio
Of all the matches left on the active roster, this is the one that needs to happen the most. Whether it's WrestleMania 28 (the ideal choice), SummerSlam, Survivor Series, this is a PPV quality dream match with a very simple story that could be put behind it (Rey being irritated that HHH would say that Undertaker is his only challenge left, HHH saying that Rey would never be able to hang with him). What other first-time ever singles matchup between two Hall of Famers is still on the active roster? None.
And if this match were to indeed take place at the Grandaddy of 'Em All, there's an easy stipulation that may not be so predictable for us know-it-alls, especially considering that the end is near for both men. Triple H's career vs. Rey Mysterio's mask (which almost is a career stip for Rey too.)