If we're talking strictly active pornstars, I'd have to say either Flower Tucci or Alexis Texas.
Naomi Russell and Charlotte Stokely were legendary, but got way too skinny for my tastes, personally. I still respect them though and consider myself a fan.
Then, I have to mention Carmen Kinsley. She will truly be missed. And yes, I will further endorse Olivia O'lovely. She might actually have my favorite ass EVER, which would be an elusive distinction.