A bit of a sequel/spin-off post to another thread asking for least favourite mobs. However, that thread is primarily for Vanilla mobs as opposed to modded ones. For me, it is a tie between the "Dwellers" (Cave Dweller, Man from the Fog, One Who Watches, Midnight Stalker, The Silence, Starved Stalker, Slumbering Omen, Goatman, etc.) and the Komodo Dragon from Alex's Mobs.

The Dwellers are glorified random death events that are usually nigh impossible to avoid and make long-term hardcore unfun. They are essentially Minecraft's version of Nextbots from Garry's Mod. Both the average Dweller and Nextbot one-shot the player upon making contact, are nigh-impossible to avoid or escape from, spam earrape, are impossible to defeat or are immune to all forms of damage. Very bad and cheap game design for a cheap jumpscare and YouTuber reaction.

The Komodo Dragon is essentially the fusion of a Baby Zombie, Vindicator, Cave Spider, and Zoglin. It poisons the target upon contact, moves incredibly fast, has high DPS, has a deceptively long melee range, and has a decent amount of defence and HP bulk. They are also extremely aggressive towards other mobs such as Sheep, Chickens, Cows, Cats, Wolves, Horses, Pandas, etc. So, good luck searching for Pandas or Rusty Wolves with these things around or if you spawn in a Jungle. They can be tamed with a full stack of Rotten Flesh, but they will still kill your Wolves. Cats, Capuchin Monkeys and, Horses regardless. Even as a tamed mob, they are still not worth it. The Grizzly Bear and Elephant from the same mod are much more reliable and better combat mounts than the Komodo Dragon.