I'd have to say thirteen. Both of my parents were alive, I was in the eighth grade, which was, in our school, the elementary equivalent of being a senior. I felt fully accepted, in the sense that I wasn't aware of anyone who disliked me. I suppose it helped that I was related to most of the people I knew at that time, but I wasn't so much aware of that at the time (I've since learned that I am related even to many of those to whom I thought I wasn't related). Life was good.
Mid-30's. Like @Hedi Mitchell , several years were The Best for many different reasons. I feel like I came into my own in my 30's.
19 to 25-ish. A lot of changes, a lot of adventures, a lot of travel and living in a foreign country.