Close call between Klaha and Juka, since I love them both... but I think it's fair to say Klaha had much better range and depth to his voice. Juka definately has the emotion, but he's still a little lacking live (whereas Klaha is friggin awesome live).
Juka is obviously pretty young, though, and I already sensed an improvement from Dix Infernal to Nocturnal Opera. He's already really good -- one day, undoubtedly, he'll be amazing. He's got the potential.
Gackt, as far as MM went, was also good but a little lacking sometimes and a little too prissy... he's probably gotten better solo, but as I don't listen to him... well, I don't care. ;p
As for Tetsu... let's not go there.
Klaha gets my vote. :]
(You know... I've NEVER listened to Schwarz Stein because it didn't sound like my type of music, but I'm really tempted to with how everyone's been raving about Kaya's voice... I give anything a go if the vocalist is good.)