As a mark, I'd say Punk. I'd like to see Punk retain at RR, retain at EC, retain against the Undertaker at WM (ending the streak), and then go on to retain cleanly against Cena for the rest of Cena's career. Objectively, though, dropping it to the Rock is only thing that makes sense. Even though he's a part timer, he's still a legend. It'll add prestige to a long reign and make sure people remember it - at least in the short term.