Well, it's not a complete top 10, but it seems noone does, so who cares?
1) James Hetfield: the old one. Do I need to explain? He sounds awesome in, say metal militia, but also sweet and soothing in fade to black. These days he's getting a bit out of the old shape...
2) Till Lindeman: Rammstein's lead singer: awesome voice, compare "Frühling in Paris" and "Stein um Stein".
3) Alexi Laiho: Could potentialy be number 1, but he can only do 1 thing good(except angels don't kill), which is be amazing.
4) Bruce Dickinson: He's a freaking sirene! He's not higher on the list, just because of personal preference in songs.
5) Tom Araya: Slayer. He needs no introduction or explanation. He rules.
6) Hansi Kürsch: He's great, he's tough, but never get's to a point where you can't hear the lyrics anymore. BTW, he's Blind Guardian.
7) Rob Halford: Judas Priest, his singing is weird, scary and shrieky, what more could you ask for?
8) ZP Theart: Formerly DragonForce. Yeah, go ahead and laugh, but I saw them live, and his singing is pretty damn sweet.
9) Tarja Turunen: Nightwish. I don't like the new singer. She's good, but not the original. To bad she was, as far as the open letter [http://www.nightwish.com/en/article/6] can be believed, a massive greedy ***** with an annoying husband...
10) Alice Cooper. Is Alice Cooper metal? Who cares, he sings awesome. More of an honorable mention, because he's not even close to being one of my favourite bands, but hey, he's history and "along came a spider" was agreat album.
EDIT: It kind of became a full top 10 anyway...