Probably 17 - 20ish. Had gotten out of the craphole that is high school, spent some time at Uni studying things I actually find interesting before putting my degree on hold, earning quite a bit of money for that age but with no real expectation that I'd do anything responsible with it, had gotten into the rave scene so every weekend was all about the drugs and music, was in a string of crazy but still mostly fun relationships, and was in great health (or as great as can be expected, given the lifestyle). Easily the best fun & novel experiences/boring grind and shitty circumstance ratio over any other stage of my life.
11 - 14 would be a close second. Was young, living on an acreage in Qld with a pool a few meters from my front door, no responsibilities but school (which I was still able to coast through on pure intellect), spend the rest of my time reading (discovered a ton of my favorite authors here, Pratchett, Hobbs, Jordan, etc. ), gaming (EQ, then DAoC, Infinity Engine games, D2, WC3), running around the bush, in the pool, or wandering around with friends (one of whom was the usual "childhood crush I never had the balls to make a move on" story - soon after our families had a falling out and we weren't able to hang out) doing random BS.
That said, things aren't too bad now either, but it seems once you hit your mid-20's the potential for 'high points' decreases sharply as expectations and responsibility build up, and the best you can hope for is a balanced and fulfilling lifestyle.
You can go back in time and change one decision, what is it?