When I was 17, getting my first girlfriend.
Stuff like finishing high school, university, getting married, getting first small rock band gig, first large gig, finding a faith, first car and passing driver's license, and other stuff ranks very high but in context of high school, puberty, and until then a lifetime of awkwardness with girls, getting a girlfriend was the biggest high.
I can't think of any feeling where for a sustained amount of time, I actually felt complete and totally high. The other highs were dampened with fear of loss on some level, but not this one.
Realistically, I never expect to match that same high again since I think more with my rational mind and not my teenage raging hormones which only amplified the effect several times over. I do miss the highs of my youth, but don't miss the depths of despair that can plague many a teenager, and usually over pretty banal stuff.