I liked CoD series, but it might mainly be because of story and atmosphere. I also liked Thief. Half Life had IIRC most fun AI, but maybe memory serves my wrong. So I am personally really not sure which one it was.
TF2 are a bit naive in terms of strategies, but they work surprisingly well together. To the point you sometimes have a hard time to distinguish one from a human n00b.
I know FrikaC made a cgi-bin version of the quakec interpreter once and wrote part of his website in QuakeC (LordHavoc)