ECC
Skeletron
For me it would have to be the newly added Nebula Floater's. I have absolutely no problems with anything else in the game, I find Moon Lord to be a cuddly little kitten compared to this beast.
The fact that he floats around anyway, shoots lasers at a decently fast rate with a good velocity which are shot not from the center of his sprite but rather his little eye minions around him makes it hard to predict the lasers path and to top all that off he teleports either at weird times for no apparent reason or in perfect time to dodge your attacks. Two of these usually always drop me from 500 hp down to 100 and I just cannot figure their AI.
A honorable mention would be Armored Skeletons early hardmode because of that nasty Broken Armor debuff that lasts for good 5 minutes.
How about yours, feel free to even make a list if you feel that motivated
Nebula Floaters and the Moon Lord. I can easily kill all the pillars besides the stupid Nebula Pillar (solar is probably the easiest in my opinion), but the Nebula monsters are annoying. And the Moon Lord himself, I struggle against him with Solar Flare armor.
Nebula floaters, Brain Sucklers, and Crawltipedes. Those things are vicious, and being a high mobility kind of player, annoying.
Nebula Floaters for sure! Then the little dudes that run fast on the ground with the brain suckers, don't remember their name. I'm playing expert ranged so when I'm up flying through the air shooting, the little bastards are so fast they are RIGHT there where I'm going to land, s!
Break LOS. A dirt box will force them to try and come straight at you. If you've got a wallhack weapon(like vilethorn, death sickle, etc) then you should be able to just hit them and force them to teleport away and try again. Done correctly it's possible to go the entire tower without taking damage, although I've never personally done a perfect run. A few will get you, but(at least for me) not enough to kill me or require more then a single health potion.
BATS CAN DIE! Oh, and crawltipedes.
If anything, it has to be those nasty Crawltipedes. I can go through the entire game, barely dying to any of the enemies, minibosses, bosses, or events, but as soon as a Crawltipede shows up, I'm nearly screwed.
CRAWLTIPEDE.
Definitely those dang Crawltipedes! When i'm fighting the solar pillar they always ruin the fun!
Crawltipedes are beaten by staying on the ground. This can be done with a floating block. Any grapple, jump, wings, or mount not immediately touching the ground is 'fair game' for them. They wont touch you otherwise unless you're unlucky to be in the path when they spawn. If you must be flying, do it in very short stents, or be capable of going much much faster then them. Vortex booster(?) may be capable of this. Mechanical minecart can, however it's acceleration eaves much to be desired.
Yes, them. Cheep trick, and no other mob deflects. It pretty much throws the ranger class out the window. You CAN do it as ranger, but you for trying.A tie between the new big mimics, and the... Selenians? Basically, anything that reflects projectiles honestly. Taking 450 from my own Lunar Flare is painful.