There's nothing like working out with someone who would also make fun of working out. Which is why Saitama wins this round (One Punch Man).

Saitama is an immensely powerful hero who can destroy any villain (and the world) in one punch. In between his monster fighting, he lives a normal life wit no cares in the world. He rarely takes anything seriously, and would make working out strange and unpredictable. Plus, there's only four things that I'd need to do if I followed his routine.

Honourable Mention:

Edward Elric (Fullmetal Alchemist)

Imaizumi Shunsuke (Yowamushi Pedal)