Nanase Aikawa is the only person these days that I can listen to a full day without getting bored.

Weekly Top Artists
Sunday May 14 - Sunday May 21
1. Aikawa Nanase 322 plays

I also recommend Mika Nakashima(<3<3), Crystal Kay, Jolin Tsai(Chinese), and mink(<3)

Of course, it really depends on taste. I say try them all and see which suits your liking best.