I love Doom (1993), and its modding community which is easily the best I've had the pleasure of being involved in! Doom itself is much more simple, fast paced, and arcade-y than other FPSes, which is what makes it my favorite. Later FPS games tend to be more slow and atmospheric with complex stories. Thanks to the game being made open-source in the late 90s, the engine has been updated by the community to be capable of just about anything, which is how we got total conversions like Sonic Robo Blast 2 and Adventures of Square, and oddball mods like MyHouse. But I personally prefer when mods stick to the FPS gameplay and bring a new aesthetic, like the beautiful Japanese Community Project, Ancient Aliens, Struggle, Auger;Zenith, and Eternal Doom (not to be confused with Doom Eternal). Compared to most other games, Doom's developers give their modders a lot more respect, including bringing on community members to make official expansions for the game, both back in the 90s and as recently as this year.

There's a compilation of all of the 90s Doom games that is very cheap on Steam, and if you'd rather not pay, you can download the game engine for free (using a source port like DSDA-Doom) and combine it with Freedoom, a fan game with identical gameplay that is compatible with most Doom mods. It's a must play IMO.