I liked Thief 3's AI, which reacted to it's surroundings and communicated with other AIs. For example; if a guard saw you at kitchen, on top of an armoire; he'd first acknowledge that you really are there, understand that he can't catch you from there(some furniture was unbreakable), alert other guards and run to get an archer/mage. They also reacted accordingly and performed thorough searches on the adjacent rooms, if you had managed to dispppear(as usually was the case) during the time that guard took to get support.