For bands like The Beatles or The Beach Boys where more than on member sang lead, each singing member will be eligible. Solo rockers (David Bowie, Ted Nugent, etc.) are ineligible, as they technically never fronted any band. Roy Orbison and Bob Dylan both sang in the Traveling Wilburys, but are far more well known for their solo careers.
http://www.ranker.com/list/greatest-rock-frontmen-of-all-time-v1/gamedemon24,
Bob Dylan
Brian Wilson
David Lee Roth
Freddie Mercury
Jim Morrison
John Fogerty
John Lennon
Mick Jagger
Paul McCartney
Robert Plant