Here you go.
Here is a chart of people's favorite/liked japanese artists in 3 big places in China which comes to check popularity.
Yellow: Shanghai
Green: Beijing
Red: Guangzhou
This is the source:
http://news.searchina.ne.jp/disp.cgi...0411_001.shtml
(thanks for Ayumiko who brought it earlier to the forum)
The top 3 are:
1. ayu
2. hikki (with a big difference behind)
3. namie
We also have to take in calculation that ayu performed already 3 times in Hong Kong since 2007, twice in Shanghai and 3 times in Taiwan (which is another country) in big arenas. The only asian country which I think ayu isn't as popular in is South Korea but I don't know much about it maybe it's half and half there (though it's not a music buyer country anyway).
On the 2nd spot it's hikki or namie.
hikki because she must be selling records better than namie in Asia but it's also not 100% since namie is the one who went to Taiwan a few times in a part of her tours and performed in small halls so it's hard to tell...
Those 3 (ayu, hikki and namie) are all popular in Japan.
kumi is only popular in Japan and South Korea and I don't know how much sales are given to her there.