Your worst enemies.
What is the enemy you just hate fighting or scare you the most? Sometimes we have irrational fears of things and often in games there is one enemy I encounter early in the game which is hard to kill then later when I am powerful enough to cut through a dozen of them they still scare me. lol.
I hate fighting the Dread Zombies. I don't know what it is, they just seem to regenerate and it takes me forever to kill one and if there are other things attacking me it's really difficult. I also am creeped out by zombies and catching their diseses!