For me, it was Avara. An online multiplayer FPS in 1996, when 3D graphics and the internet still felt like science fiction. The skill ceiling was high and games were very competitive. Shout out to the ®ed §quadron, the clan I was in. I am still in touch with friends I met in the game, more than 20 years later.
I loved Avara. The level edit capability offered so much freedom.
You can find my first level, “chut” out on the internet. My other, better levels, are lost to me.
I reverse engineered something I saw in a demo level, where they took doors, triggered them to “open” both vertically and horizontally and set them at the half way point to create “corner” pieces.
No other maps had these pieces, and I was fascinated with the idea of using them to create terrain. I ended up making a “quilt” like structure of ramps and corner pieces to make a unique arena. I had not taken any upper level geometry yet so I was just doing it by trial and error. Fun times.
I also found you could import wav files and trigger them so I made a jukebox level, which was absolutely massive (uncompressed audio!).
I remember sharing these maps by trying to lure people to join my server, but transferring them could take some time because I was on 26.4 modem at the time.
Avara was probably my first “development” experience... amazing.
The game has been ported to modern OSs: https://github.com/avaraline/Avara/
There is a video of gameplay with commentary here: https://www.youtube.com/watch?v=AutG8KO4DsY