Kelly Clarkson. She can actually sing, so he won't have to work on the voice for hours with autotune to make it sound listenable. And she actually gives a **** about the lyrics, if she doesn't like it, she ask for a partial rewrite...
Though his work with P!nk rocks too and she's an awesome singer as well.
idk probably the one he has most control over.
Katy and Ke$ha write their own material so they're not as easily controlled by him as the others are although I am aware that they're still very much under his wing. Those creepy rumours about him being bossy and rude