Justin.TV during the first 3 years or so. Honestly, because we were in "cowboy mode", and that's what I enjoy the most. The site was growing and changing so fast it was often hard to keep up, and we were all just doing whatever it took.
I also worked in a hypergrowth unicorn from the early days and I would also call those first couple of years there my happiest job, even though it was tons of work and full cowboy mode.
That was when I was in my mid-20s and single. I am 100% sure that I wouldn't enjoy that right now in my current stage in life. (mid-30s and married now)