The Oklahoma co-ed may have looked a little different when she made her American Idol debut on Season Four – the hair a little curlier, the clothes a little frumpier –but her star power was certain. Even honest curmudgeon Simon Cowell knew it, guessing that if she won (she did), she'd be the highest selling American Idol contestant in history. He was half right: she went on to be one of the highest-selling country artists, period, beginning with her 2005 debut Some Hearts. Now a Grammy winner, a CMA Awards co-host, a huge touring success and member of the Grand Ole Opry, it's hard to measure the massive impact and influence of one of the genre's biggest stars. And, never one to forget her roots, she still is a devoted Idol fan, returning to the show nearly every season since.