I got to thinking about this and I thought it might provide for some interesting discussion.
Producers work with any number of popstars, but certain popstars draw out their best work for some reason. Ones that come to mind are Max Martin/Britney, Dr. Luke/Katy, Ryan Tedder/Any big-voiced diva, Bloodshy & Avant/Sky, and Stargate/Rihanna.
What is it about these popstars that draws out the best in said producers, when sometimes all they're providing is a persona and a voice to the track? There are surely quite a few examples I missed as well. This could easily turn into a discussion of how the music industry works when it comes to how artists choose producers or how songs circulate.