Arachnotrons. I don't care what anyone says, because they have their own caves, which usually plots you in the middle of the level, completely surrounded, and then they all converge on you peppering you in a neverending cloud of plasma. With Revenants and Mancubi, I can atleast outwit them and use tactics and stuff. There is no strategy in arachnotron caves besides raw desperation tactics and being VERY well-prepared.
Now, when revenant/mancubi caves get implemented...