I find few things more frustrating in a game than when you have an AI-controlled character that, intentionally or otherwise, does his/her utmost to sabotage your efforts. I can't think of too many examples off the top of my head but ones that spring to mind are (1) Noober in Baldur's Gate. Who hasn't killed this guy? (2) Lance in Vice City Stories. Although he is characterised as crazy, is he supposed to be so crazy that he constantly veers into your submachine gun fire during one of the motorcycle missions? After you end up inadvertently killing him you just want to resurrect him so you can kill him again. (3) Any of the NPCs in Sacred if you had to save them. The unarmed fools would charge the nearest enemy and slap them with their fists until they met a swift and deserving end. Pain.
Which NPCs would you give the honour of being the most moronic and frustrating?