13. Gwen Stefani, "Hollaback Girl"
"Hollaback Girl" was the beginning of an era. As the first song to sell a million copies as a digital single, it shot to Number One on the Hot 100 shortly after Billboard began factoring iTunes sales. And Gwen Stefani's flirtation with hip-hop swag ended up being a harbinger of an era in which rap-savvy caucasian pop stars like Ke$ha and Iggy Azalea routinely top the charts.