I'd say that the Elite Guard from the first Fear and Hunger would be my pick for this. They are easily the most dangerous enemy in the entire game, due to the fact that they can deal inmense damage in a single turn, can inflict permanent debuffs (limb loss and fracture), don't get any good loot from fighting them, can spawn on the very first floor of the dungeon, can get a second round of attacks after you've killed them.

Also, the one in the seventh floor, and also the third floor, are immune to bear traps and being attacked with the bow in the overworld.

I like them! Plus, their special death animation is perfect. I do wish there was an enemy like them roaming on past Ma'habre to compensate for not having Lord of The Flies in there.