Carrie UnderwoodAfter nearly nine full seasons of winners, finalists and hopefuls, 'American Idol' has produced some of music's hottest stars. And now we know which of the show's alumni reign as most successful in terms of combined album sales, individual song sales and radio airplay. Billboard has officially ranked the 24 most successful 'Idol' stars ever, and not surprisingly, country music makes a strong showing.

First season winner Kelly Clarkson comes in at the No.1 spot. Known to both pop and country music fans (thanks to her hit duet, 'Because of You' with Reba McEntire), the Texas native boasts more than 10 million albums sold and radio plays exceeding four million, making her show's most successful star thus far.

Right on Kelly's heels is two-time ACM Entertainer of the Year, Carrie Underwood. With over 11 million albums sold, she has the most sales of any 'AI' star, and will quickly surpass Kelly in both album and track sales if her golden (and platinum) streak continues.

Rounding out the top five are Chris Daughtry at No. 3, teen sensation Jordin Sparks in the fourth spot and David Cook at No. 5. Country darling Kellie Pickler comes in strong at No. 7, while recent finalist Danny Gokey comes in at No. 22.

The hit TV show has not only helped launch country-music careers, it has also played host to several country acts, including Shania Twain, who served as a mentor to the contestants, and Rascal Flatts and Keith Urban, who have performed on the show. Recent 'Idol' cast-off Aaron Kelly has announced plans to pursue a country career.