Yeah, all Cena did was make the feud matter. Remember, without that Cena feud, Punk would be lucky to be in the dark match this past WrestleMania instead of fighting The Undertaker in the co-main event. No 434 day title reign, no "best in the world" gimmick. That's what Cena brought to the feud. Relevance. Not to mention excellence on the mic, as always. That has to be the most hilarious misuse of the word "owning" I've ever seen in my life. Cena more than held his own.
I digress, because that's not my answer. I think Punk may well be the answer by the time their respective careers are over, because they still have some feuding to do, but not yet.
The Randy Orton and Edge feuds are very close here. The feud with Randy Orton lasted longer, partly because it was interrupted by Cena's injury, and involved more matches. More gimmick matches. Because of that injury, they really had two distinctly separate feuds that were both quite memorable. But I'm going to go with the Edge feud for two seasons:
1.Randy Orton is Just. So. Boring. Yes, he was given a great gimmick that did its best to cover all his flaws, but at the end of the day the flaws are still there. Generic on the mic and in the ring, it's hard to imagine him being anything more than a mid carder if his last name's not Orton and if he wasn't placed in Evolution to get the rub of all rubs from Flair and Triple H.
2.This could be a topic that takes up an entire thread(anybody else can feel free to make it), but in addition to all the great moments and matches they provided, IMO the initial Edge-Cena feud did as much to elevate both of their careers as any other feud has done for the participants in the history of wrestling. Before the feud started, Edge was a midcarder trying to break through and Cena was a fresh faced main eventer trying to establish himself there. Eight months later, when the feud came to an end(for the time being) in that TLC match, Edge was a bonafide main eventer and Cena was the face of the company. That feud proved Edge belonged in the main event and it proved Cena could carry the WWE. That's what separates it for me.