This question is difficult to me, because of the difference between singing and growling/screaming. They both take inordinate amounts of talent, but they can hardly be compared. I'll just list my 5 faves for each style:
Singers:
Bruce Dickinson (Iron Maiden)
Michael Kiske (old-Helloween)
Edu Falaschi (Angra)
Matthew Barlow (Iced Earth)
Tobias Sammett (Edguy, Avantasia)
Growlers:
Angela Gossow (Arch Enemy)
Mikael Stanne (Dark Tranquillity)
Tomas Lindgren (ex-At The Gates, ex-The Crown, The Great Deceiver)
Mikael Akerfeld (Opeth, Bloodbath)
Satyr Wongraven (Satyricon, Wongraven, Storm)