led zeppelin influenced every rock band in the world. that is a fact.
no they didnt, Jimmy Hendrix was hard rock and he sure as hell werent influenced by Zeppelin
Purple and Zeppelin shared a mutual respect and inspiration for one another even though Purple went onto a harder and faster sound.
As for Sabbath they were influenced of the likes of The Who, Jimi Hendrix, the Yardbirds, Cream. They have never listed, as far as I have seen and I have seen most of the documentaries that exist of Sabbath, Zeppelin as on of their influences
If we are going to talk about underrated and underappreciated hard rock acts, lets talk about Trapeze. They made three with two very good albums before bassist/vocalist Glen Hughes left to replace Roger Glover in Purple