Nana Ogura (小倉奈々) has quite possibly the best butt in Japanese porn. It's so round, and so big in proportion to her body. I think the shape is just perfect. On top of that she usually sports tan lines to make that ass look even more delectable.
Even from the front you can tell what a nice butt she has, just by the way her hips flare out!
Here are some good butt shots that I found in an image search:
Trust me, it looks even better in action!
EDIT: Here's a page that has all the photos I posted here, and lots more!
http://bblike.blog.fc2.com/blog-entry-116.html