There's great stuff to be found in them all, but there's something about the music of the first half of the 20th century - 20s, 30s, 40s and 50s that I find very calming and relaxing. Even with two major wars and the great depression, they truly were simpler times.
It's fascinating how, when you begin to listen to popular music era by era you get a better sense of the cultural context and you can hear the social stress level gradually increasing over the years as if it were an actual audible background noise within the music itself.
Equally interesting is the pop culture explosion in the 60s, when everything change literally overnight. You can hear the rise of rock'n'roll throughout the latter fifties and into the early sixties as a gradual evolution. Even the appearance of the Beatles wasn't a radical change at first. 'I Wanna Hold Your Hand', 'She Loves You' and "Love Me Do' weren't a huge departure from what groups like The Beach Boys were already doing. But then Bob Dylan introduced them to pot - and everything changed. The opening note of "I Feel Fine" with the feedback guitar was the shot heard round the world. And that influence is still with us today.
Oops. Obsessive rant, sorry.