I agree with Johnny, while
Sammy Hagar is a superior singer,
"Diamond" Dave(in his heyday of the early 80s, of course) was Van Halen's- and one of Rock's- best frontmen. Still, for longevity and consistent quality, no one surpasses the likes of
Roger Daltrey, Robert Plant, Steven Tyler(save for a few years in the 70s), and certainly
Mick Jagger.
Bob Dylan is a good choice, too, if a bit unconventional. The sadly departed
Micheal Hutchence of INXS also comes to mind. Growing up in the 80s, I can attest that he ruled the pop scene in the latter half of that decade. I'd also add
David Gahan of Depeche Mode to the list. He's a pure frontman- I don't think he's written any of their songs(at least not the significant ones); that's been mostly Martin Gore's job.
Kurt Cobain tried to make himself out to be the anti-frontman, in accordance with his general rebellion against "mainstream/corporate" rock. He failed miserably, of course, in his tragically brief career. When you've penned most of the music and lyrics in the group, and have such a naturally intense stage presense, you can help but be considered a true frontman.
Lastly, not unlike- and perhaps even more than-
Paul McCartney*,
Sting was a frontman whose ego grew way too big for the rest of The Police. So he went solo- to the detriment of his music, but the betterment of his back account.
*I hope I don't ruffle any feathers by saying this, but Paul McCartney is ultimately responsible for the break-up of the Beatles. Sure, the talentless mistress of screech, Yoko Ono may have played a part, but it was Paul's reaction to her, his bloated ego, and his prominent position that precipitated the group's needlessly permanent disintegration.
Cheers.
-AlphaLima