Left 4 Dead 2

Valve's 2009 co-op shooter is four survivors, a route, and an AI Director that decides what happens on it. That last part is the design: item placement, horde timing and special-infected spawns are generated per run, so a campaign you have played twenty times can still ambush you. The four specials from the first game are joined by five more, each one a specific answer to a specific player mistake - the Charger punishes clumping, the Spitter punishes camping, the Jockey punishes wandering off - which means the game is really enforcing teamwork through threat design rather than through objectives. Melee weapons and incendiary ammo were added, along with campaigns that end in genuinely inventive finales, and Versus mode lets four players take the infected side. Sixteen years on it is still populated, largely because the community workshop is enormous and Valve handed over the tools. The AI Director remains one of the most-cited pieces of procedural design in the medium.