In Poland for example, I hung around Warsaw University during the daytime and wow!, there are some tall beauties with the big bubble butts like you describe. One such girl I saw who was maybe 5'10" I saw in the university library had such fantastic hips and a perfect wide round ass in some tight brown pants that I had to talk to her. Unfortunately, she said she did not speak English when I asked her in Polish, or if she did she was either too shy to speak to me. A warning is that Polish is a brutal language to learn if you are a native English speaker and it is not as commonly understood as in Scandinavia or Western Europe. However, there were many other girls with great asses that you can find all over Warsaw that have a knowledge of English, including pubs and the whorehouse places like Rasputin and Studio Relaks. One such night I went to Rasputin and for US$40 had an encounter with a tall beautiful black-haired 27 year old Polish girl. She had fantastic hips and a nice round butt, in addition to a big tattoo scene that covered her back. What a view! Hitting that big ass doggystyle with that big tattoo scene on her back! A great memory!
As to your ranking question: I will have to think about it some more, but for sure, of all the "white" countries, Poland is number one. If I had to pick a single country for the best and biggest asses I guess it would have to be Brazil.