Native Artists:
Current, IDK.
Overall, Utada or Ayumi.
Utada's on a break so she can't be biggest right now, but I'm voting for her anyway based on the sheer hugeness of her sales.
You really need to do your research before you make a thread like this. It's completely ignorant of the Japanese industry's actual state and throwing in Utada, who took a break years ago, doesn't fix the fact that all the other acts are Western.