Fear AI was awesome back in the day, the way they would flank, traverse obstacles and become aggressive on queue with their radio chatter was very impressive.
A part of me wants to say STALKER, though I think that was more noises through the environment or using certain equipment. Not sure if it was due to mods but I've seen some stuff in those games that I've never seen anywhere else before.
Two distinct memories I have are of watching two squads of opposing factions face off in a firefight, using lots of cover, throwing grenades at each other and trying to flank. It ended with one side all dead bar two guys left on the floor bleeding out. The other squad disengaged and started looting the corpses and healing each other, except for one member who walked over to the wounded, picked up their dropped side arm, switched to it and then proceeded to execute them both with their own gun before casually walking away. It was properly brutal and shocking to see from an AI.
The second was me creeping through Red Forest early into a sandbox game on the anomaly mod. I had no night vision and had to use the flashlight sparingly to avoid detection from some nearby bandits and monsters. I finally made it through, stopped to plan my next step in the map screen and was suddenly blasted down without warning from behind. I thought I must have walked past somebody in the dark right next to me and completely missed them. I loaded the autosave from moments earlier, turned around and the guy was actually tracking and sneaking behind me waiting for an opportunity to kill me. I loaded an even earlier save from just after I passed the bandits and he was still there, he must have been hunting me for a good 10-15 minutes, waiting until I was clear of all the other potential enemies to strike.
