I like Doom 2, Duke Nukem 3D and Blood. Cultic is also great!
I also play Wolfenstein: Enemy Territory (WolfET) sometimes. WolfET is free (both as in freedom and free as in beer) class-based multiplayer FPS game.
You can get WolfET from:
* Steam
* SplashDamage site (needs ETKEY from http://etkey.net/)
* or, you can use ET: Legacy client (recommended).
ET: Legacy has QoL enhancements (that don't break compatibility with original vanilla WolfET) and better support for modern GPUs. You can get ET: Legacy from the official website or you can use Flatpak. You want to get the 32-bit/multilib version because most mods are 32-bit only. If you use vanilla WolfET, you need to use the in-game console to set a var to make it work with modern GPUs: \seta r_primitives 2 (open the console by pressing the key under ESC). ET: Legacy doesn't need this fix, but other versions of WolfET require it. You can disable vsay audio by running this in the same in-game console: \seta cg_noVoiceChats 1
Useful links:
* https://github.com/etlegacy/etlegacy/wiki/Useful-Links
* https://antman.info/wolf/cvar/
* https://et.trackbase.net/serverlist/
* https://et.splatterladder.eu/?mod=serverlist
In addition to those, I also play occasionally TES: Daggerfall (in DosBox), Morrowind/OpenMW, NetHack (with X11 GUI and tiles), Dwarf Fortress and Shattered Pixel Dungeon (available as a flatpak package or F-Droid app).