I think that Rihanna would make for too much singing, Iggy for too much yelling and pop, and Nicki just right for making a real hip hop/rap collab. The others options would be intersting though. #ThePinkPrint (including Beyonce, Jeremih, and Ariana Grande)

Rihanna because I love a lot of her music and it is more likely that Katy would collaborate with her because they are good friends! Also I don't like rap too much and I can't Nicki Minaj. I never heard Iggy so I don't know about her music. I Think Rihanna would sound best with her out of these choices anyway!