As this is a rock supergroup, a powerhouse vocalist is required. Technically ability is less important than being able to cope with cutting through the almighty racket that would no doubt be coming from behind them one stage.
The late Soundgarden front-man was not only a big lunged screamer but could also provide some rhythm guitar stability when needed. If you don't think he could cut it in such exalted company, listen to "Black Hole Sun".
However, I've moved on from my original thoughts. A supergroup needs a super front-man, especially when you consider the planet sized talents already included. A great singer, obviously, but also someone with the charisma and star power to hold not just a stadium audience in the palm of their hand, but a worldwide TV audience of an estimated 1.9 billion people. A great songwriter who has penned some of the most iconic songs in rock history. Someone whose band had so many hits they had not one, not two but three "Greatest Hits" compilations.
The one, the only Freddie Mercury.
Anyone who doubts his stadium filling charisma should check out the "Live At Wembley" or "Rock In Rio" footage.
His songwriting chops are not in doubt, with multiple number one singles. As a bonus he could play a pretty decent piano too, although hiding him behind a keyboard is a waste in the live context.
So the great Freddie Mercury completes my line up of Keith Moon, Paul McCartney and Jimi Hendrix. Anyone have a better suggestion?