Personally, I don't think there's a decent ass among the bunch, and here are a few pics to prove my point:
Hana of Dallas
JinHee
Original Penny
Yoko
Sue at Smile
I just wish I had more pictures to show you. But as you can see, none of them are really very appealing.
Seriously though, it's all relative isn't it. Certain ethnic groups are known for certain physical characteristics, so if you're looking for butt among the Asian ladies, you may have a long (hard) search ahead of you. But man, what a fun search it will be!
Hope this helps you in making your decision. Any of the ladies mentioned above provide outstanding service. If you see any of them, they will make you quickly forget what you were really looking for in the first place.
Whatever you decide, it won't be wrong; whether you see one of the beautiful AMP providers or one of the great indy ladies, you will be forever hooked.
K.