Had to go with Hansi in the end. He's so much more eclectic than anyone else on the list but Allen. Hansi also has that completely own sound of his. That powerful near-growl thing he can switch on and off on the fly. From that voice I always get the vibes that he sings too loudly into an old microphone that makes the voice crack, which added much in John Fogerty's or Eric Burdon's raspy voices for example. Except that he doesn't, he sings with modern equipment.
I'd love Kürch to cover I Put a Spell on You or The House or a Rising Sun for example.
This voice is all about power and what seems to be a tough training, because he wasn't much of a singer when he was young. And this is just one of his many sides. His high register these days is outstanding; he's even gotten that aggression for his high-pitched stuff on At the Edge of Time that pretty much lacked still from A Twist or ANatO. He sings that way even though it doesn't seem he could do it. Also, he's a master singing calmly in mid register (ballads). During this he still sounds very powerful btw.
He's just a perfect singer to make the aggression appear or disappear out of nowhere. He's good at interpreting just about anything, except I think he can't be anything but loud.

His voice is so very powerful.
What counts most though is that I prefer his voice(s) over those other singers, except maybe Kiske whose high register for me is magic and, tied with Mercury and Kürsch, hearing his voice pleases me the most. ^^
Others worth mentioning in the list for me are Russell Allen, Timo Kotipelto, Andre Matos... and even Kai Hansen for his sheer unlikeliness and charisma.
Also, no Lande, Barlow, Hayer, LaBrie & co. on the list? This is some huge Finno-German-Brazilian scene only. Plus Allen who's as much a power metal singer as LaBrie would be, wherefore I mentioned him, too. I'd imagine he could be the favorite singer for many people, too.