Kaleidoscope

My first choice would definetely be Hizumi (D'espairsRay), his voice has exactly the kind of tone I prefer and he has an awesome mixture of emotion and technique.
Technique-wise I would say Gackt, no matter what people think about him, he is an amazing singer and has the most powerful vibrato I have ever heard outside operatic music.
Others would be:
Daisuke Ochida (Kagerou, the studs)
Ryo (9GOATS BLACK OUT)
Tatsurou (Mucc)
Satoshi (girugamesh)
Toshi (X JAPAN)
Hatis Noit (Mutyumu)
Ryuutarou (Plasti Tree)
Satsuki (Rentrer En Soi)
Otogi (Awoi)
??林太郎 (although his voice is not really "clean" ;D)
There are certainly lots of good singers outside of VK, but tbh not many come to my mind right now, maybe also the guy of The Novembers.

Pretty much this, I think especially Gokusai works very well, most people not familiar to Japanese music liked it when I played some songs to them.