In no particular order:
1. Russell Allan (Symphony X) - Absolutely amazing, I'm surprised not many people have mentioned him yet, he is incredible!!
2. Eric Adams (Manowar) - The manliest man in the world, and the power in his voice is just unbelievable.
3. Bruce Dickinson (Iron Maiden) - epic, lots of high notes and a top notch live performer.
4. Dio (Dio) - Because he's awesome, HOLY DIVEEEEEEEEERRRR!!
5. Roy Khan (Kamelot) - The man is absolutely unbelievable: http://www.youtube.com/watch?v=IWDALIOzXNk&feature=related - nothing more needs to be said.
6. Tobias Sammet (Edguy) - The next Bruce Dickinson, but with possibly more power in his voice.
7. Rob Halford (Judas Priest) - THE METAL GOD, Painkiller is amazing.
8. Ihsahn (Emperor) - He sounds so evil, listen to Misanthrope off his solo album angL, and prepare to shiver with fear.
9. Dani Filth (Cradle of Filth) - an odd choice I know, but he is so versatile and unique. There's the high squeals, the low growls and several really individual styles in between, he's really effective at establishing an atmosphere.
10. Michael Akerfeldt (Opeth) - He's really versatile, he's got those brutal growls, and then he's got that those really nice clean vocals.
There are of course, plenty of other fantastic metal vocalists - it is the best music genre as fare as male vocalists go - but these are just my favourites. If there's anyone here you guys haven't heard of I sincerely reccommend you check them out coz they're AWESOME!!!