What retro FPS has good co-op? I played through Duke 3D with

Hexen or Hexen 2?

DOOM and DOOM II co-op kicks ass.

Playing System Shock 2 with a buddy is loads of fun, we both planned our builds beforehand and covered each others weaknesses.

>>2922168
seconding this, though it stopped being a survival horror the moment you realize you can shoot your friends off.

File: unreal.jpg (274KB, 1000x625px) Image search: [Google]

274KB, 1000x625px

>>2922072
Unreal has a pretty good co-op mode.

>>2922072
Coop is pretty arse in 90's FPS. Some Doom wads would be good for it but only using modern rules. Classic coop involves either playing levels easy enough on single player becoming really boring with more than one or hard levels where you die and since ammo doesn't respawn you are stuck with the peashooter forever.
Just as boring whether it's easy or hard.

Honestly I feel that Serious Sam is the first FPS ever to get coop right.