Looking on happiness alone, I'd say primary school. Everything was easier then - far less anxiety, no OCD, 3-4 fairly good friends, good achievements in school boosting dopamine, playing Pharaoh and Gruntz on my parents' computer...
But I guess the best overall (despite some mental health issues) was the period when I was studying two majors. Being financially privileged, in a country with free higher education, I could postpone adulthood and learn something interesting by studying cognitive science in addition to computer science. I could choose very interesting subjects and "interesting" is one of the top values in my dictionary. It also quite literally changed the way I think about many things. I knew down there that I'll become this software-whatnot at some point, but it was good to have some vague dreams of a career as a scientist or philosopher for a moment. And I learned about analytic philosophy, and that we don't know s**t about mind and brain, and that there are more than 20 definitions of the word "innate", and that a system with two light bulbs is conscious according to some, so it was worth it.
Almost the same q., but slightly modified:
What has been the best or the most interesting period of your life so far?
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Recently diagnosed with ASD, still skeptical.
Feel free to PM me if you want to or are really bored.
Either overthink or don't think at all - there's no middle way.