Akula said:
The clone soldiers from the original F.E.A.R. I remember they had solid AI and would surprise me during every fire fight.
I'm going to agree with this. F.E.A.R.'s AI was very responsive to situations, and were capable of not only taking and using cover, but using the game's physics system to create their own cover where none had existed before.
The enemy AI in the Half-Life games also works together very well and is very well programmed, but it intentionally places the enemies in places where it's possible to shoot them. I wish there was a harder difficulty setting where they did their best to keep themselves out of the line of fire, rather than a harder difficulty setting where they just do and take more damage.