I'd go with Britney Spears. She is still massive even now. There's still photos of her at least once a day in some major newspaper.
Sadly most children/teenagers these days actually don't know who Madonna is, which is why my vote went to Britney. Even young children today know Britney to some extent. The screams when she performed with Rihanna at the 2011 Billboard Music Awards show that she is still very popular.
I don't know if she'll still be as popular in another ten years time, as Madonna was still pretty popular in the early 2000s, but at the moment she gets my vote as she is the only artist on that list who I feel has both career length and current popularity.
So you think that anyone born before 1980 must be dead or incapable of switching on a radio ?