It's Kelly Clarkson, and she is by far the most successful of the three AI winners. Her first album went double platinum, and her 2nd album is projected to go triple platinum by September. She's had several #1 hits on the Billboard charts, and has been the #1 female pop artist in the nation since last fall. She did a very well-reviewed turn on Saturday Night Live.
She's currently on her first solo tour, playing to sold-out houses across the country. The tour is so successful, that they are extending it this summer after she finishes the initial scheduled dates in May and goes to the UK to promote the release of her album there.
She is so mainstream and considered one of the hottest up-and-coming young artists (just turned 23), that she recently got one of the most coveted advertising gigs -- she's the newest "Got Milk" girl.
Here is an image of the debut ad and some of the copy from the accompanying news release.
In addition, for those who care about such things, Kelly is a practicing Christian who had a cross tattooed on the inside of one of her wrists because, she says, it keeps her humble.