Freddie Mercury deffinitely was the greatest frontman of all time..
Close to him (in showmanship) i'd pickled Jim Morrison, Robert Plant, Ian Gillan, Roger Daltrey, John Lennon, maby Mike Jagger, Bon Scott or Eddie Van Halen - in some ways they had something more or something less, but Mercury was the King Of Rock, all the singers who sang at his tribute concert (including Robert Plant and Roger Daltrey) said that "he was the greatest", so logic states that there can't be a better.
All of the people i have mentioned had different charismatic features, but when you rememer Live Aid, Live Magic and Barcelona sessions, it is stupid to favor someone else..