Maaya Sakamoto=
m-flo's English is not bad either.
As for BoA....when speaking English, she's pretty good, but while singing for some reason...it gets hard to understand, and she also has her Korean accent. Let's wait to hear the full version of "THE CHRISTMAS SONG" from her "Merikuri" single, which is a full-length English song. From the song sample on her official Avex webpage, her English sounds decent.