MInami's cutesy voice makes me want to murder people.
I'm not familiar with Lustknot, the sample provided sounds kinda average.
Kaya has a great powerful voice, but I do notice him going a little off key at times (especially in Freya, unless the song is just written that way, which would be odd).
My personal favourite is Mahiro, it is not easy to sing like that. He's also pretty versatile.. I was really impressed with what he did in "Tsukubai" and My Dragon's first single.
Buuut, in terms of "most skilled" I'd have to go with Ran. Perfect falsetto, wide range, and sounds pleasant. First time I listened to Xepher I was kinda blown away.
(all of this is IMO, I'm not a trained critic)