Of the games I've played, I'd have to say F.E.A.R. for sure. The AI is both aggressive and smart. The game wouldn't feel the same with a less aggressive and smart AI.
I also liked the AI in the first Crysis. I honestly have no idea why a lot of people seemed to dislike it, because I thought it acted in a pretty smart and believable way. It also supported the suit powers very well. The AI may not have been very good at combat tactics but other than that, I think it was pretty good.







