1. Hollaback Girl - Gwen Stefani's biggest hit is also her best. The mix of cheerleader stomps, schoolyard chants and riotous production grows in its bombastic nature, as its hook moves from an acoustic guitar to a woozy mash of synths and trumpets. The song is rather simple and silly, but with 100 percent certainty, no one else would sound as cool or convincing pulling off such a performance. Somehow, Gwen Stefani the solo artist successfully pulled off one of the weirdest singles of the 00s, and spent four weeks at No. 1 with it, making it the first song to sell a million digital downloads. "Hollaback Girl" is a reminder that when something is so obviously unique to an artist, people are forced to listen and undoubtedly respond.