The most frustrating monsters to encounter as a PC...besides rust monsters

[icefractal]

It really depends on if the party plays 'combat as sport' and just walks into fights or if the party plays 'combat as war' with scouting and information gathering before going into the dungeon.

I've heard this, but - it's not like the monsters can't use tactics too. "Powerful but dumb thing you can defeat with good planning" is one thing, but there are some monsters that could easily harass a party to defeat if the DM didn't take pity.

On topic - things that can stay out of LoE almost all the time - incorporeal creatures with Flyby Attack popping in and out of the walls, for example. It's an effective tactic, but a really annoying one.