I still think Perfect Dark is the best when it comes to enemy AI. They weren't perfect by any means but the amount of options an NPC has is incredibly long and you can just never predict what they will do. So many examples of an enemy reacting to your presence to shooting guns out of their own hands and seeing if they will surrender or try to shoot you in the back. Shooting out the lights leaving them confused in pitch darkness or turning on the lights if they were using night vision to blind them. You could shoot them in the leg and watch them move around the entire map limping. The AI was just insane.
I think Halo has a very good balance of AI. They can be either laughably funny or super dangerous depending on your situation. Some times Grunts run away and other times they stick you with a grenade when you weren't expecting it. The Elites act more like a Player where they will take cover, ambush you or jump out of the way of explosions. Also Infinite you will see them attempt grabbing new weapons if they see an upgrade.
I wish more games focus on AI however i can also understand that making AI too smart will deplete the fun factor especially in games where AI have access to Sniper rifles.







