So many people refer to their fave as 'The Queen of Pop'... Even the media throw the term around when discussing female artists. There doesn't seem to be much debate over the male "King of Pop" title, as much as Usher would like to change that
I'm not really a huge Madonna fan but I wouldn't try to say anybody else.
Madonna is and always will be the true Queen of Pop, while Britney is the reigning Princess of Pop. However, it is quite possible that GaGa could take it from her someday.