I've been playing Crysis 2 lately (steam sale, yeah) and it's really awesome. The game looks amazing and using the suit to play around with enemies is some of the most fun I've had in shooters. But I noticed the AI isn't that great. Not bad, just... whenever you decloak, take a quick headshot and immediately cloak again, they act as if they didn't see you even if it was right in front of them.
There's more stuff like that and although it suits the gameplay really well, I became interested in playing FEAR again. I remember that game had the best AI I've ever seen in a shooter. I went back to it and it's still the best I've ever seen in a shooter. Granted, I don't play a lot of shooters, which leads me to the question:
What's the best AI you've ever experienced in a shooter?