Rabbits due to their insane attack speed and the range of their attack, but my all time most hated is the boars, they have way to much hp, do way to much damage, and if you get a elite one in arcade mode your pretty much fucked if there is no rock to hide behind. It attacks so fast and hits so damned hard, the attack speed is so fast that you pratcally cannot get a hit in without taking one in return.