If I may opine on some of the previous choices here, Dic
kinson and Akerfeldt are getting
way too much credit. Dic
kinson is technically proficient but pretty lacking in raw power or emotional content (possibly the main thing that sets Maiden a notch below Judas Priest). Akerfeldt seems only remarkable due to the fact that he sings clean vocals in addition to death growls, and while his clean voice sounds very nice his tone is just too plain and "gentle" to really be compelling.
Off the top of my head, I'd rank these guys as among the best:
* Halford (duh)
* Quorthon (definitely for emotional content above anything else)
* Dio
Oh, and an honorable mention to Layne Staley of Alice in Chains, as AiC aren't exactly metal but Layne crushes all three of the above guys talent-wise.