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.
"In all this white, you'll see me like you've never seen me yet" Wrapped in Red - Kelly Clarkson
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