Definitely second half, because many genres I like, started to develop by then. AOR, New Wave, Post-Punk, and Synth-Pop by the end of the decade. Also Progressive Rock and Hard Rock were at their peak by then.
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1st for me:
Led Zeppelin, Deep Purple, Black Sabbath, The Who all produced some rockin albums in the early 70s.
Though I'd cry if I had to stop listening to Fleetwood Mac Rumours from '78.0