The AI in Call of Duty 2 and 3 (and I'd imagine in the previous games, though I never played them). They rarely hit anything, but perhaps more frustrating is the fact that they always wait on you before they advance anywhere. This fits the definition of 'useless' pretty well. If you just sit behind cover and wait, they will just stay where they are, dying frequently and occasionally killing an enemy. If they succeed in killing all the enemies, they'll just sit there some more. They're there to make the game feel bigger, more like a real war, but this behavior may actually make it feel smaller, since you have to do all the fighting.
I also think that Halo 2's friendly AI can't be the most useless, because it's not even the most useless in the series. I actually think it's the most useful in the series, because they actually have weapons that they can use effectively, and fight enemies that they can fight effectively.
Now, Halo CE... THAT has useless AI, because no matter how they fight, they are usually equipped with a weapon that is only effective at spitting distance and does very little damage to the Covenant. When they finally fight an enemy that will actually die when they shoot it, it's an enemy that often kills them in one hit.