I don't like it when the AI knows everything, especially when that everything includes my exact location.
WoW has perhaps the most extreme example of this, with mobs able to know your exact location, no matter how many corners you go around, no matter how far you run, up until the leash distance at which point that run very very fast back to exactly where they started, regardless of how far or how confusing the route they took. Just to top it off, some don't even care about vanish, such as many undead, and more than a few bosses. Or that's just the perpetual AoE which seems to exist for the sole purpose of unifying two groups who would otherwise share nothing: healers and rogues.
Oblivion takes a more moderate route. If you're sneaky enough, enemies will have an idea that you're out there and possibly have the general area, but won't perfectly zoom in. There are, of course, exceptions. And by exceptions I mean every single pet/companion in the game, whether always there or summoned, which will make a beeline for the player, no matter how well hidden. Oh, and often times the summoning NPC will still be entirely clueless.
Stalker NPCs are not so informed, but much smarter. They don't know exactly where you are, but they will be able to figure out your general location, which is when cover becomes important. Then they start searching. They don't take a straight route. They might take different routes, making groups dangerous not just for their higher total health and damage output, but for their ability to attack from two directions. But in keeping with their ignorance, missed bullets, even without a silencer, cause no reaction at all. Maybe they think they are heroes in action movies who can never be hit by bullets, and so if they are flying past, it only confirms their theory.
I'm probably being too hard on WoW. It does seem harder to make an AI not know everything. It's simply a lot easier, and less error-prone, to allow a NPC to know your location than give it the intelligence to guess. Come to think of it, as far as I can recall, except for specific stealth/invisibility mechanics, it's only recently that I've run into many games with ignorant-but-smart AI. Though that comes with two qualifiers: my idea of recent is the past 5 years or so and I was in the gaming closet for a long time, meaning that I had a pretty small library until the past few years.
In an old classic, Escape Velocity, the ship AI not only followed your exact location, it would aim by this method, which unless it was using a beam attack, almost guaranteed a miss. On a side note, ship turning was almost perfectly linked to the graphics (technically no, but let's pretend), which were based on square grids of the ships pointing in the different directions, given a typical grid of 6x6 or 8x8, there were quite a lot of desired firing angles which were impossible. On the other hand there was Starcraft with it's ignorant-but-stupid AI which seemed to love nothing more than chasing lone, fast enemies into fun places like siege tank emplacements or fields of lurkers. Clear foreshadowing for the eventual development of World of Warcraft and tanks dragging mobs into a bunch of AoE-happy mages. And a decent bit of countering evidence for my theory that ignorant-but-smart AIs are a newish development. Still, they do seem to have spread. A welcome change. I definitely prefer an AI that has to figure things out. It's so hard to trick God.