I'm kind of answering this late, but my favorite Japanese singers/bands are:

-HIGH AND MIGHTY COLOR (One of my all time favorite bands!)
-UVERworld (Second favorite band, as opposed to them as many people are.)
-Do As Infinity
-HOME MADE Kazoku
-L'Arc~en~Ciel
-Malice Mizer
-KAT-TUN
-Hitomi Takahashi (Incredible singer!)
-Hikaru Utada (What can I say? Awesome!)
-Ayumi Hamasaki
-Hyde
-Gackt
-BEAT CRUSADERS
-Nightmare (They are great!)
-Maximum the Hormone (They make me laugh!)
-Nana Kitade
-Anna Tsuchiya
-Tommy Heavenly6
-LONG SHOT DISTANCE (Started with them because of Naruto Shippuuden.)
-AKFG
-YUI (Incredible singer!)
-Younha
-Stance Punks

That's my list of favorite Japanese singers/band.