What about impact though?
Whitney sold less than Celine and Carey, but is arguably the most influential singer of the three. Madonna is arguably more influential than them all.
That's why I say there's no definitive "Queen of Pop".....what set Michael apart from his peers was his unparalleled success, impact and ultimately his influence. That's why he's the "King of Pop". With the ladies, there's no undisputed Queen because there are several women that have a right to the throne. They've all been called the Queen.