Civilization 3 is so dank and addicting
You can play it for months and not really want to do anything else
The complete edition puts in some nice expansions too, lots of native people's are in it plus it adds badass wonders like The Knights Templar which spawns a strong a** crusader dude every five turns where you built it, which puts the computer on the ropes like immediately
It has lots of great mods and the community is kind of active, multiplayer servers are still up on steam too I think
Also the music is really good :v even the caveman song is the absolute weed

Serious Sam. It's not as popular or successful as doom or as much of a memetic failure as duke nukem, so this series is just kind of obscure compared to the other "classic" shooters.

Sam is phenomenal. top notch boomer shooter

Oh I thought of another one: Far Cry Instincts: Predator. A 360 bundle release of the two original Xbox Far Cry games that I played hundreds of hours of because it had an extremely expansive map editor and custom games. A lot of Halo customs I played were based on those, although i cant say for sure they weren't already game types people made in like quake or something