Sports games anyone? I think the way players move and act in a game like Fifa ist pretty amazing.

This is true. AI in FIFA, while it certainly has its flaws, is really underappreciated.

The Last of Us HANDS down.

During stealth the AI is somewhat easy, I imagine it's due to frustration otherwise and that is why the game is best played WITHOUT listen mode.

Once hell breaks out =>

It continues to surprise and I never forget amazing moments like the video below.

How the enemy communicates with others and you. How they flank you, generally act scared of their friends lives being in danger, protecting themselves. Now add the fact that your companion is calling them out, throwing bricks, jumping on others, commenting when you bashed someones skull in... It's so fucking dynamic, never seen anything quite like it.

Killzone 2 & 3 have great AI too but it doesn't touch TLOU in my opinion.

Killzone 2/3

KZ3 AI were quite good, I thought. They moved from cover to cover and tried to flank you. You could pin an enemy down but then you'd have one come around the side if you weren't careful. If they were entrenched, then you'd often have to move to a better position yourself.

It's amazing how they never fall off a ledge!

Galactic Civilisations 2

imo of course,...

other valid candidates mentiond in this tread: Red Turtle from Super Mario Bros 1 (dat shitty animal got you always) and Chess.
In the last case that credit would go to Houdini, developed by Robert Houdart.

demon's souls.

Killzone 2/3, The Last of US, and F.E.A.R.

Halo (only thing the franchise has going for it imho)
FEAR
Rage

I do think a lot of the great AI in Halo kind of fell off a cliff moving to this gen from the last. None of the Halo's I have played on 360 can come close to matching the AI from the Xbox games. But the design of the game and its almost sandbox type setting really gave the AI a ton of room to operate. Which is why I am also impressed by the second two shooters, they manage to pull off great AI while being the typical corridor shooter. Meanwhile CoD/KZ/BF and even the TPS like Gears/Uncharted/Last of Us are all shit in comparison imho. You play those games 10 times and the missions play the exact same way 10 times out of 10.

I don't game on PC all that much but I am sure there are plenty of strategy/sim games that have outstanding AI as well.