Probably the generic enemy soldiers in the COD games. They just fucking drop like flies. You could ambush them with a broken vodka bottle and they'd be slaughtered.
1. Miriam from Alpha Centauri. She was the biggest fucking moron to ever exist in a video game. 2. A.I. troops from ARMA II. Totally have to love the insta-headshots through 10 meter thick-as-shit grass at 300 meters with Ak-47's. 3. A.I. Civs. in Civilization V. Predictible and Pathetic failure measured by the metric shit-ton.
Total War games don't have shitty enemies, they just have horrible and buggy AI. Does Mount & Blade count? 'Cause I did. If it doesn't, I suppose EUII has some pretty fucking retarded AI.
Beat me to it. Cliffracers are so bad, it was part of Elder Scrolls lore to have them chased out of Morrowind by Jiub, the dude you met on the boat, and he became a saint because of it. Cliffracers have my vote as worst.