There are probably plenty of current porn stars who have great asses, but I'm just not into porn as much as I used to be back in the 80's and 90's. Sorry to all the newer gals (especially the ones contributing to this site).
Some of the older guys on here will remember Stacey Donovan. She was around back in the mid-80's and into the early-90's. She had the nicest ass I've seen on a porn star. A close second would be Racquel Darrian.
Stacy:
http://f.imagehost.org/0495/Stacey_Donovan_04.jpg
Racquel:
http://i.imagehost.org/0597/Racquel_38.jpg
I like them to have narrow hips, and no flab on their outer hips and thighs. I do like a nice bubble butt, but it has to be small. I'm not at all into huge asses like those of J-Lo, Kim Kardashian, etc.
Simply put, an ass should be round, but it should be round and small. Just stuffing a bunch of cottage cheese into the trunk makes it round, but it also makes it look out of shape and old, and frankly just fat and sloppy looking, in my opinion.
I know a lot of guys like girls with thunder thighs and junk in the trunk. I'm just not into that. I guess it's good that there are so many different tastes.