Popular music is tough to define, besides the obvious qualifications: Does the song have mass appeal? Are people talking about it? Is the track glossy and polished? Does your mom tap her feet to it? Answer "yes" to at least three of these and you've got yourself a pop song. As the 00's winds down, we're in a nostalgic mood and looking back at the decade's best pop artists, from Britney to Beyonce. When it came to the Top 40 set, this here was a great time to love music. Though we know you'll have serious problems with it, here now are our picks for the Best Pop Artists of the '00s: