I think that award would have to go to Rihanna. She started her career, bare belly and all, in 2005. 8 years later, she's still showing her belly. And it's not just when she goes to the beach and wears the smallest of bikinis. She wears cropped tops in public, during her concerts, at award shows, etc. Not to mention sexy magazine spreads she does, the times she is part of her country's national parade, and the Instagram photos that she posts. Not surprising that she so comfortable in her bare skin. Since she was raised on a tropical island, living in a bikini and walking around half-naked was probably a way of life to her.
My runner-up would have to be AnnaLynne McCord. She has shown her bellybutton in nearly every movie she has been in (including an extreme close-up), she wore underwear or bikinis in nearly every episode of 90210 (including the Princess Leia bikini), she's always wearing bikinis at the beach, she does sexy magazine spreads from time to time, and she has been spotted wearing cropped tops in public. The only thing I have against her is that she prefers the word "navel" over saying "bellybutton".