BattleMaster was my greatest success in player counts. Ironically, it started out as a small strategy-game add-on to another game, SpellMaster - hence the similarity in names.
It quickly took off, then surpassed its parent, then left it in the dust. At its peak, BattleMaster had a few thousand active players. It spawned an active Wiki and several spin-offs. I could barely manage the load of running the game and during a few years had to open up new game worlds rapidly because the existing ones kept filling up to capacity.
It was intense, it was fun, it is now over 20 years old and still going, though years ago I handed it off to the community to keep running. For a while I was still providing the server and database, but even that has now moved elsewhere.
Still, I’m quite proud of it. BattleMaster was a fully automated game and yet quite social. Over the years, at least one marriage has resulted from people meeting in-game and several families have played together, sometimes staying in touch through the game when real-world circumstances moved them apart. I consider these things a huge success.
See battlemaster.org for the current incarnation of the game.