First campaign setting–published or homebrew–that you played in.
The day I checked the PHB out from the library, and held it in my hands, I had–as was so often the case–to go sit at the local rec center for several hours. My brother was in all the sports, my mother therefore coached all the sports, and my dad was into racquetball. I would happily have been just about any place in town that wasn’t the rec center; even the bowling alley had better video games than a flickering Punch Out! machine. But I had the book, the sacred book, so it was irrelevant.
I read about three chapters and then wrote my first custom race. I had arguably been preparing to do that for most of my childhood, and already had pages of rpg worlds and character classes made for Breath of Fire-style theoretical games. So the first campaign I played in was homebrewed, as was the second, and I DM’d them both. I joined a homebrew campaign run by the friends I made after skipping a grade, and we pretty quickly (I think we made it two sessions?) decided to swap it over to a game I ran. That campaign went for several months, then there was a weird interlude where my best friend and I started gaming with some college kids–and I usurped that campaign from them as well. That was my first time playing in something published, specifically Dark Sun, and I loved everything about the setting. However, the DM let me roll a cleric of Set, and then kill a beholder (which he threw at us, I think, because he didn’t really know what else to do in that adventure) by dropping a Wall of Force on it. My argument, which I knew was entirely spurious, was that since a Wall of Force theoretically has no thickness it must have infinite mass (“Like a black hole,” I said, all of 14 years old) and therefore deal infinite damage to the beholder while bypassing its antimagic eye and falling automatically because I cast it without any anchor points, parallel to the floor. I took his acquiescence to these things (I mean, a cleric of an Egyptian god in Dark Sun?!) as a sign of weakness, and pounced upon his materials to claim them as my own. DnD!
He and his group actually seemed happy to relinquish the control, though, and we actually had some great games in Dark Sun. I loved the lethality of the setting and the ubiquity of the psionics. A lot of my time was spent converting normal monsters to weird Dark Sun variants–there were the giant wasps that looked like that armored thing from M.C. Escher’s Curl Up, but they had a bunch of human fingers down their sides that they could elongate into whips with the relevant psionic power; some dogs with chameleonic powers I used to ambush their caravan; and this awesome carapaced Dragonne I actually built from clay and had one of the greatest encounters of my young gaming life throwing at the party.
We never really dipped our toes into any other official DnD products; we technically played in Greyhawk with the advent of 3e, since that was the default setting for that system. But, as is true in all hobbies since, when I get my hands on a thing I usually need to make it my own to begin understanding it. Still have a soft spot for DS, and have returned a few times, but I’ve never had any real hunger to play in the Forgotten Realms or Dragonlance, even if I’ve consumed quite a bit of their respective fictions.