It really depends on the criteria. Are we talking strictly vocal chops? Are we talking charisma? What about a combination of both? Gregg Allman is one of the greatest blues singers ever. Ronnie Van Zant wasn't a great singer but he was a great lead vocalist. Hell, Ozzy is a great lead vocalist even if he can't sing worth a damn. Overall I would go with Freddie as far as the guys go. With the females it doesn't really matter what criteria you use because it's still going to be Ann Wilson.