I'll second Natalie Merchant, I'm surprised she was only mentioned once so far. How bout a duet with her and James Taylor? Now that would be smoooooth. Kate Bush is my favorite female performer although I don't know if she quite fits in the "most beautiful voice" category. She could open a can of song writing woop ass on just about anyone though What about Chrissie Hynde, a unique and beautiful voice I don't think was mentioned.