Pretty simple question.
1969 and 1970 were the only two years where my 3 favorite bands existed at the same time.
So for me, I'm debating on whether to go with 60's or 70's. I love a lot of the mid to late 60's, but a lot of the popular stuff before that was kind of too old school style for my tastes. The 70's had some great music as well, although I don't care as much for the Disco/Dance/Punk style that became the dominant part half way through.
I'm going to go with 70's, because a lot of great work seemed to be accumulated in that decade, and most of the stuff I'm into from that time isn't dated at all.