The original Doom for sure. I don't know if they count as mods since they're technically running on a modified version of the engine, but there is still a massive community making maps, mods, and even new game modes for it. Most recently the "MyHouse.wad" map for it has exploded in popularity. I've had so much fun over the years with doom mods, it's a treasure trove that most people have no idea exists.

There's even entire new games built on Doom. Sonic Robo Blast 2 Kart (stupid name, I know) is a fantastic free open source karting game with a decent community, and it's technically just a doom mod.

If anyone here is going to play a single doom mod to see what's up, I recommend Doom: The Golden Souls Remastered. It's good fun.