I think Russell has the best overall metal voice, because not only is it powerful, dead on key, and really involves you in the music, but Russ really knows how to USE his voice, accent works, where to use vibrato, where to slur. He's an absolute pro. In terms of natural voice talent, Edu of Angra is downright incredible. Such a big full sound comes out of that skinny guy with almost zero effort, he's like the male Whitney Houston, only without all the domestic abuse (or so I would assume anyway). In terms of knowing how to layer, produce, and inflect vocals, however, Kai Hansen hands down. Just listen to Somewhere Out in Space and try not to fall into a huge outerspace pirate war smuggling laser fantasy.
I have always thought Jorn Lande sucked. Granted I haven't heard him in a while but when I saw him with Yngwie I was mega disappointed. He seemed like an amature in tone, presence, pitch, everything. Then again, following Mark Boals and Jeff Scott Soto is not so easy.
BTW - best voice ever is Freddy Mercury