The MCBusted star, who became a father in March (14), has topped the Father's Day-inspired Marvel poll.
Cowell and athlete Mo Farah were both shortlisted for the prize.
Fletcher says, "I feel very honoured, especially considering the competition I was up against. I apologise - sorry Simon Cowell and Mo Farah!"
And talking about his son Buzz, he adds, "Buzz has completely changed my life. Even before he was born, when we found out we were having a baby it just completely turns your world upside down."
The pop star documented every day of his wife Giovanna's pregnancy by posting touching videos online.