I might get chewed out for this, but the best female R&B voice I have ever heard belongs to Whitney Houston. Nobody has a voice like hers.
The best male R&B voice belongs to Donny Hathaway. The man had range, soul, everything. No competition. The males listed couldn't even sing a Donny Hathaway song. The same applies for the females listed and Whitney Houston songs.
I don't know why anyone listed Lyfe Jennings as a great vocalist. His voice is all over the place. It's different, and it fits his songs, but it wouldn't make him a great vocalist. It actually hinders him from being that. Don't get it twisted I'm a Lyfe Jennings fan. I'm just being realistic.