Toeing the provocative line between loneliness and desire, Britney Spears became a national obsession with 1999's “...Baby One More Time”, and doubled down the following year with “Oops!...I Did It Again,” which exemplified the bubblegum pop formula she developed alongside Swedish producer Max Martin. But the pop star grew up before our eyes on seductive tracks like “Toxic”, and a more mature artist has since emerged, with cuts like 2011's “Till the World Ends” demonstrating the former Mouseketeer's ear for epic hooks, as well as a vulnerability that is as compelling as it is rare.