Mine:
Dio
Bruce = Halford
Blackie Lawless.
Klaus Meine
Bruce/Halford... They both have a lot of versatility, and both have a lot of power. There are many things that I just could not see Bruce singing that Halford has, and vice versa - Such as I could never hear Halford doing anything with the variation of Jerusalem, but Bruce doing Painkiller would be an equally unlikley thought for me. I don't think either is "better". Dio is the same in this regard but, maybe just because I'm on a Dio kick lately I don't know, there's something about his voice. All three are close and all are fantastic at 'brutal' metal aswell as some fantastically beautiful ballads.
Blackie, well he can do some really heartfelt and really heavy stuff, and is another one capable of great screams like Halford but I place him 4th on my list because I think he's a bit less diverse. Sure he has a great range of styles and I love them all but there are things I CAN imagine Bruce/Halford doing and not Blackie.
Klaus has a fantastic voice but doesn't suit the heavier stuff so well so he comes down the lower of the 5 for me.
Other notables imho include Alice Cooper, Tim Owens, Axl (As above posters have said, many different voices to him none of which I really deem metal or as strong as Dio/Bruce/Halford but I still love the voice) and in the same manner as Ozzy - Dave Mustaine and Lemmy. They may not be amazing 'singers', but their vocals sure fit the music.
In "General" vocalists, Freddie is ofc attached to the top of the list, and then I have a real shitstorm as I try to work out where I would place people like Steve Tyler.