Kelly Clarkson is still the most successful American Idol winner, beating out Carrie Underwood, according to a formula released by Billboard. While Underwood is on top of her genre and has sold more albums than Clarkson, Billboard still gives Clarkson the edge. Kelly Clarkson Most Successful Idol - Beats Carrie Underwood, Billboard. According to an … [Read more...] about Kelly Clarkson Most Successful Idol – Beats Carrie Underwood, Billboard