[Shaun]

Queen: Madonna. The title was invented for her. No one's close to replicating her career and discography.
Princess: Britney.
King: MJ.
Prince: I don't think anyone's ever referred to a prince of pop, but I'd probably say Usher.
Duke: Bowie, since he had the character of the Thin White Duke whilst in his pop phase.
Duchess: Again, not a title I've seen used but idk... Diana Ross maybe?

I don't see the need to give every current popstar a title. Some of them are going to end up as the next Dido, Lil Kim and Christina Milians.