I may have overlooked it but, I don't think, and can't believe no one's said Aretha Franklin yet.
anyway, SOME great voices that I can think of off of the top of my head are:
-Nicky Hopkins -( I think that's his name, he sings "Fooled around and fell in Love" for Elvin Bishop and "Jane" and "Save Your Love" Starship)
-Aretha Franklin
-Whitney Houston (I didn't say I liked her music)
- Leanne Rynes ( whatever her name is)
- James Brown ( It's a man's world, Come on, this cat's wailin' on that track)
- Big Mama Thorton
-Robert Plant
- (singer for) Soundgarden and Audioslave, I'm drawing a blank on his name for some reason.
- Sheryl Crow
-Patsy Cliene
and here are some of my personal favs, (not necessarily great voices, I just like the way they sing):
--Dr. John
--John Lee Hooker
-- Gregg Allman
-- Juvenille ( I think this dude has a great voice, I'd love to hear him sing some reggae, or just sing something, period)
Oh well, I know I'm forgetting about a million people so, there's my short list.