Billboard has spoken and they deem Kelly Clarkson as the most successful American Idol star. The magazine ranked former Idol contestants, like Clarkson, Carrie Underwood, Jordin Sparks, and David Cook, on the list of Idols Top 24. Keep reading to find out why Clarkson ranked No. 1 and which other stars made the list According to Billboard magazine, Kelly Clarkson, the first aspiring ...