I'll discuss each time period, and then give my verdict.
If you're talking about the most important time for music, I'd have to say the mid-to-late 60s, just for sheer influence. Even if you just take the Beatles and the Beach Boys (the two most influential groups of the 60s, and arguably of all time), it would have to rank tops. Then you'd have to include Bob Dylan, Jimi Hendrix, The Yardbirds(who may have had a shit singer, but were important to the development of what would become heavy metal), the Stones (a *real* rock band), ect.
The 70s...probably tops when it comes to diversity. You had your metal, you had your disco, but if you think that's all the 70s produced, then you're wrong. The Eagles were huge during the 70s, and rightfully so. Hell, Rod Stewart was actually good in the 70s before he descended into adult contemporary. Late 70s too was great, with punk and the beginnings of new wave.
The 80s happen to be my favorite, for what mudda and Johnny said. Plus the fact that I grew up in the 80s, so I may be kind of biased.
The 90s were good until the latter part of the decade. What a lot of people seem to forget was that during the mid 90s, there was a bit of a retro-trend, esp. towards the 70s,between the grunge era and the start of rap-rock hell (which was GREAT in theory, but it meant we got stuck with poseurs like Fred Durst).