I would give it to AJ, nobody has been more impressive to me this year. But if I'm thinking the way WWE would, I can see them giving it to Ambrose and I as much as I may want to, I can't really argue that decision.
Maybe because I soured on him a bit last year I didn't really notice, but he had a phenomenal year. (You decide if the pun was intended)
He was in the final two at the Royal Rumble, for the first quarter of the year was heavily involved in the World Title picture, he faced Brock Lesnar at WrestleMania (albeit in an underwhelming match). From there he went on to feud with Jericho, then at MITB he won MITB and cashed it on the same night to win the World Title, he beat both other members of The Shield at Battleground, for the majority of his reign was the only World Champion in the company and after dropping it to AJ in September, he remained right in the middle of the title picture until the end of the year.
Not to mention like AJ, he beat Cena clean (granted it was a 10 minute match on Smackdown... I think)
You can argue that he benefited a lot, especially for the first third of the year, from the injury bug that plagued WWE late last year... but regardless, that's one hell of a year.