I believe this has come up before, but the one enemy that I like the absolute least are Lava Slimes. There are plenty of enemies with unfair attacks like Necromancers or are just plain too strong like Nebula Floaters, but none of them actively grief your
, which Lava Slimes do.
I usually end up making my cave town in Hell as a replacement for the hellevator and to not worry about it getting eaten by biome spread, but I have to seal the houses in with hoik doors and fill it with background walls so lava slimes don’t come marching in and tearing out the furniture. I have to tear holes in my Wall of Flesh bridge so that the lava will drain (i’m not using platforms - then the bottom eye can shoot you). I have to micromanage where I kill them so I don’t wall myself into a hellhouse with a pool of lava. There is no other single enemy that requires you to work around their existence so much, and it’s because someone thought it was a good idea to make them spawn a
flowing liquid on death.
And even then they can still mess you up. The lava can rip up Wall of Flesh campfires, get rid of pots, and once a Voodoo Demon conveniently died right over a freshly killed Lava Slime.
Point being, I hate them so much and it’s a stupid idea to make an enemy that can grief you.
This!
And similar things!
Although, I wouldn't say lava slimes right away, if you're not a summoner, you can usually just ignore them. if you're a summoner though, your minions will cause you a bunch of problems killing lava slimes, and ruining stuff...
But some other enemies that affect blocks and stuff are similarly the worst, (IMO)
3.) snow balla. that one litters the place with snow blocks everywhere, even where you can't reach. (like random underground holes you didn't even know they existed there). they often trap themselves too, and just place a bunch of snow everywhere. it's a bigger pain to clean up after them, than the whole frost legion being any difficult.
luckily, they're limited only to frost legion event.
2.) goblin peon. they dismantle doors, ruining your housings with them, so NPCs get confused and move away, and you have to put back all the doors, and housing flags after you turned the goblin army into minced meat.
you can prevent it with having all doors open, but then the whole army marches through your houses. or if you fight the army farther from the town, but if you go back to the town (or die and respawn there) they'll quickly dismantle your doors everywhere where they can reach.
1.) meteor head. although they don't ruin builds, (meteor crash-sited may do though, or just ruin the terrain in general) they're probably one of my most hated enemy. they don't drop anything in hardmode, but they don't really drop much pre-hardmode either (meteor, yeah, but the whole biome is full of that). they can be killed with a single hit with almost any weapons, but they hurt a lot if hit you, and they go through everything.
they often appear in large swarms, perticularly in the jungle, evil biomes, etc. (or so it seems). where spawn rate is increased. you might have like 6-8 of them on the screen the same time. and new ones spawn even before you could kill them all.
they replace everything else that can spawn in the area (except harpies and wyverns), even including event enemies, and they ignore things like towns too.
and if you want to get rid of the biome, sometimes you end up having to kill 700-800 of them by the time you mine out the entire charash site, if it landed in the jungle or something.
you can place X amount of meteor ore blocks somewhere or leave a crash-site or two unmined (if they're not in an annoying place, but they're often in the most annoying places) so no more meteors will fall, but generally meteor heads are just an annoyance. it's not a very happy thing, when the game tells "you killed the 1200th meteor head!" and you're actually just trying to mine out the 2nd crash site because it's right next to a town or something...