We may as well call this our personal favorites thread, as "the best" is always going to be subjective. Ones I didn't see on this thread:
Ann Wilson from Heart has got to be near the top of the all-time list. Just about the perfect rock and roll voice.
Mavis Staples deserves some strong consideration, although "most unique" would probably be a better descriptor than "best".
Etta James has to be on it, as "At Last" is one of the best recorded performances ever.
Bonnie Raitt isn't for everybody, but for a mellower bluesy sound, I'm not sure there's anyone better.
Carole King, Dionne Warwick, and Karen Carpenter all merit your vote, if you're in more of an easy listening mood.
I could go on, but then I'd be competing with the FAST FRED style (RIP, my friend).