Gawd, the Halo ones were certain to be mentioned, but they seriously aren't that bad compared to other games... How about Rainbow Six? Movement AI is non-existant, they simply follow the paths you lay out for them, and shoot enemies when they're nearby. They have no way of getting out of being stuck if you put waypoints wrong, and I've found enemies running around in circles, who stop to shoot me, then when I'm dead they continue their running around.
The original Timesplitters. It never got much better, but the original was horrendous, with groups of enemies following the same paths, charging at you in lines, and staying in those lines while they shoot you. This problem was never fixed in the map maker either.
The friendly AI in FEAR are pretty shit too. They just get killed or run away every time shit hits the fan! Stupid idiots!