I feel like Duke 3D was probably the first mainstream accessible moddable FPS. Doom of course had plenty of level editors, but Duke Nukem brought the ability to alter and script AI as editable plaintext CON files, and of course any skills you learned on the BUILD engine were transferrable to any number of other games (Shadow Warrior, Blood, etc.)
Also shout out to anyone who remembers "wackplayer" - Duke's equivalent of the BEEP keyword.
Duke also had a map editor with a 3D editing mode. It allowed raising, lowering floors and ceilings, and picking textures. Ahead of its time. The complexity of brush-based true 3D really put a damper on good, built in editors.
Also shout out to anyone who remembers "wackplayer" - Duke's equivalent of the BEEP keyword.