60s and 70s... i merged the two since they somewhat blend the classic rock genre. some of the most talented individuals and bands came from this time period. all-time legends like zeppelin, allmans, jimi, srv, the who, pink floyd, zappa, the yardbirds, jeff beck, big brother and the holding company, the grateful motha****in' dead!!!!! i long for yesteryear when musical creativity and complexity were still highly valued...
to the OP who said the 90s because there wasn't much copying of music... you have to be joking, right? the entire grunge/punk/alternative/heavy metal scene was perpetuated by the big guitar bands of the 60s and 70s, who were in turn perpetuated by the original blues sounds of the 40s and 50s... and so the cycle goes. ALL modern (20th century and beyond) music is a deviation of some earlier form of music/spoken word.