Hard to generalize in decade terms. I tend to think of popular music's greatest periods as being shorter and more transitional. So to me -- the late 60s/very early 70s (the Beatles completely altering popular song form); the late 70s/very early 80s (punk, early new wave and the beginning of post-punk overwhelming hippiedom; and -- very important -- MTV), and the late 80s/early 90s (the beginning of hip-hop; grunge destroying hair metal; lo-fi and DIY) are all periods where certain bands were making major significant transitions after certain musical styles had become bland with lots of poor quality imitators.
I freely admit I know little about popular music since then. And I can't speak to rock's early transition from the blues in the 50s.
I can find shit and genius in every year and every decade.