Quake and Quake 2 have a bunch of co-op modes and they have been updated for crossplay on modern systems. I was just running some of the newer map packs with my buddies last night. Quake is $4 on Humble right now.
New classic Doom versions have very good split screen options
Personally i like Duck Game for parties of 4+
I love Gooseman’s work, especially Tactical Intervention. Talk about “ahead of it’s time,” there isn’t a game today that can match the vehicle “optional” extraction maps in that game. Stupid fun community too.