for me it’s not necessarily one monster type specifically, rather it’s how combinations of monsters work together to be particularly annoying.
eg. spectres + gelatinous cubes. can’t target the spectres directly becuase of perma-stealth, so you have to kill the g.cube first, which has tons of HP’s and can’t be CC’d in any way (auto restore), meanwhile the spectres are gleefully one-shotting your whole team with their stupid reach attack (spectral grasp) that also pulls the target which deals extra damage.
you pretty much have to pray for a shevirath or saarvin “hit everything” proc on their basic against the cube so that it hits the spectres too and kills them with collateral damage.
speaking of reach attacks. the pull attack on the troll + multi-hit seems to do an absurd amount of damage. So for a single enemy type, they would probably be the ones I find most annoying, but at least they can be CC’d.