As a fan, CM Punk. Typically the great champions have been ones that ushered in change to the show or a new theme. Hogan in the 80's, Stone Cold/The Rock in the 90's. Cm Punk now.
I don't count Cena because he never really brought greatness to it. I know he was playing his role but there is nothing about any of Cena's runs that stand out. Just youtube CM Punk promo. His "shoot" promo was something not done in the WWE for a long time.