#5 John Cena

John Cena's case is peculiar. He started as a heel and slowly transitioned into becoming a babyface and ever since has remained there and never turned heel. There has hardly been any divergence in his character and it has more or less been the same throughout.
There has been clamouring in the past by the fans wanting to see John make the switch but it never happened (arguably because of the effect it would have on the merchandise sales).
But in many ways when you build a character around the slogans of "Hustle Loyalty Respect" and "Never give up" you can assume it would be really difficult to make a switch to the dark side and make it work (The chants "Let's Go Cena, Cena Sucks" might help though).
Even then it would be safe to say that maybe for someone with Cena's wealth of talent and ability, it was worth a try. Wasn't it?