For me it's Hanna Hilton right now. She's got the most beautiful face, I think. And her tits are fantastically big and natural, while still being a slightly 'imperfect' shape in a way that you don't really get with fake tits, and that I think makes them ten times sexier. Plus, since she started doing boy/girl scenes it turns out she fucks great too!
Kira Kener is a close second, for all round beauty/tits/ass quality. But her scenes can be really boring sometimes.