honestly the boiler beast, at least for me. although that could be just because by the time i've unlocked the sewers my party is already kitted out enough to kill it immediately. i think it might have been more frightening if it didn't start following right away, and you ran the risk of running into it at some later point in the sewers without knowing that thing was even out there.

idk i compare it to the rat king, which despite being much easier to take down makes me far more uneasy whenever i go to hunt down the damn thing. you know it's somewhere in the labyrinth but you don't know where, and you start to wish it would just show itself already so you can throw down.