by far, the summer after high school graduation, in 1999.
I spent a month in Europe with my friends, had a cushy 9-5 job as a file clerk, and spent tons of time hanging out and partying with my high school gang. I also spent most of the summer in love with 3 different people, which was nice. Didn't end up with any of them, but there's still something nice about feeling such a strong emotion for so long of a time.
The only real downside was that the drama reached a high point by the end. My best friend and I got into an all-out fist fight / brawl (5 years later, and we still aren't speaking to eachother; it takes some effort to even get us in the same room), and another good friend cheated on his girlfriend (another really great friend of mine), with my other best friend's girlfriend.
But all the drama happened at the end of the summer... Before that, the summer really rocked.