I'm not really a Queen fan, but I have to say that Freddie Mercury was hands down the best frontman there ever was. If there's ever been a person who was just born to be on a big stage of front of a hundred thousand people, it was him. I've never seen anyone work a crowd like he did at Live Aid. That was pure magic. He had the crowd in the palm of his hands and enjoyed it almost like it was something sexual. Amazing.
One of the best frontmen I ever saw in person is a guy called Ray Cappo. He used to sing in an amazing Hare Krishna punk band (yeah, you read that right) called Shelter. That guy was so charismatic and intense, it was just unbelievable, because for him his music was literally a religious and spiritual thing. During the first years of the band, he was actually a monk and he was really going off on stage like there was no tomorrow. Too bad that later on he turned into a jerk who drank, did it with groupies and ripped people off while still pretending to be a dedicated Krishna devotee. Me and a friend once went to four Shelter shows in one week and after the last one I went up to him to tell him that I'd seen them four times that week and that they'd been amazing every time, and he just said "Oh, OK" and walked away... :-/
This is him when he was still cool:
(sorry for the bad sound)