I'm really inclined to say the AI in Metal Gear Solid 2 and 3. They acted and behaved like normal humans with specific patrolling schemes; they were clever enough to respond to certain situations doing a specific course of action (that mostly gave you the disadvantage or got yourself killed), they were always curious about something weird on the enviroment (using a wetbox to hide indoors already give all the suspicion in the world), they responded to player imput, whether sound, a sneeze or irregular footsteps, command would call from time to time and if no one responded a team would be dispatched to check upon the situation; you could hold-up enemies and interrogate them, and they would mostly behave differently (there were the cowards who would just beg for their lives, or hardened soldiers that wouldn't scare until you start torturing them), and they had even secret, normal dialogue between them from time to time. In fact, the least human-ish thing about these were their patrol itself, which was a very game-ish route (from A to B, rinse and repeat), but getting spotted would made hell break loose, calling command and an armored team would then confront you, while other soldiers went to get some cover and support them. They'd use grenades if you made a fort in one room, and avoid/kick you if you get too close, followed with a point-blank shot.
Honestly, it was a truly advanced AI, one that rivals games from these years. On several aspects it felt like the AI from MGS4 and 5 took a massive step-down in some of the most simple aspects (Getting spotted in MGSV inmediately makes everyone aware of you and full alarms, whereas getting spotted in MGS2/3 by a single soldier would raise an alarm until you take that specific soldier out, or he calls for command and gives the full alarm). It's crazy how evolutionary most games have got, yet a sofisticated AI seems always problematic. But for some reason, MGS2 and 3 really nailed it with the AI, and while it can certainly be exploited (like every AI, let's be honest here, if you were trying to pull that monkey video on me for the comment), I can honestly point at it as some of this industry's finest enemy AI.
