Cena.
Him at the top made me hate WWE for over a decade and every time he got the belt, I either stopped watching or at least tried to skip his segments as much as possible. I get having the ultimate good guy hero babyface champion, but it went for so long and so many times that I became impossible for me to sit through on a regular basis.
Roman is, at his core, a monster heel champion and that usually lends itself better to longer reigns as you wait to see who will topple him and that makes the stories better. The storylines for the Bloodline last year until this year's Mania season was better and more entertaining than anything I saw Cena do. That said, Cody losing at Mania kind of took the wind out of the sails as I am now wondering who they have left that I would want to see beat Roman at this point.