From 69-73 they put out some very good albums. They were kind of the leaders of hard rock. IMO the others you mention are not Hard Rock stars, they're rock stars with a wide range of sound. I guess those who followed hard rock, Deep Purple was kind of the original band. I never became a hard rock fan. To me Tull, Zepplin, The Who, The Stones continued to break new ground and grow. While I love Machine Head, I don't think it compares with Zep II, Thick as a Brick, or Who's Next which came out around the same time. Machine Head is the only CD I have of theirs and it is on a very limited play schedule because I only like about 1/2 of it. I need to get Live in Japan at some point, but that would also be in pretty limited play schedule. If I want to listen to heavy rock I listen to Tull, Who, or Zep usually. Those who would turn to AC/DC, Black Sabbath, Aerosmith, etc would probably prefer Deep Purple as that is what their sound sort of became. Hard driving guitar jams.