I'm in college now, so I'm pretty young, but I've listened to music from a lot of different eras, and I've always been particularly fond of the 90's. Grunge is my favorite kind of music, so naturally the early 90's would've been heaven on earth for me, which I guess makes me biased. But hell, even the hip hop from the 90's was tolerable.
But I've also gotta give credit where credit is due. The 60's and 70's brought along so many revolutions/trends in the music industry that both decades have to be up near the top. So personally, 90's is my preference, but I'm very thankful for what the 60's and 70's brought. The Beatles, The Stones, Hendrix, The Who, Zeppelin, etc.