BONNIE PINK is amazing. I think her english is pretty good. Not perfect but she has a pretty good accent compared to most japanese artists out there. I mean, she sings whole original english songs in some of her albums.
Rope Dancer and Wildflower are good examples.
Wildflower:
http://s6.yousendit.com/d.aspx?id=A0...832F8F766C4D9E
Rope Dancer:
http://s6.yousendit.com/d.aspx?id=91...7414D571871587
What About Me?:
http://s6.yousendit.com/d.aspx?id=AD...4CCEA48A5AB5FE
DoublE has pretty great and understandable english imo.
Crystal Kay obviously has a fluent english. She released a whole english cover album.
OLIVIA, I think she intentionally makes her english not clear because of how she changes her tone of voice so constantly over her songs.