Best would probably be 63-73, but we'll count it as the 60's
Worst is a tough call between the 80's and 00's. Yes, there was good music during those decades, but it's vastly overshadowed by 80's disco and 00's pop.
I'll vote the 80's the worst though.
90's was pretty awesome, but part of that bias is because that's when I was growing up (and listening to grunge/alt/hip-hip). I'm assuming that's why the frikin 80's are getting some positive votes...