There aren’t many monsters that I dislike. Dire weasels are just odd to have in any book anywhere. Unicorns are never used. None of these I actually dislike though. I suppose any arachnid would qualify – a Colossal Dire Spider would cause me to promptly wet myself, and I’m not a massive fan of Driders. But those I’m just afraid of. I don’t dislike them as monsters. Driders are pretty cool, really.
No, if we’re looking for the monster which I actively dislike as an entry in the monster manual, it would be the Duergar. What mind thought them up? “Well, we’ve got evil elves with the Drow, so we should totally make an evil dwarf race too for the Underdark. I know! We’ll give them weapons in their BEARDS! Like… needles. Poison needles. That they shoot out of their beards… totally makes sense. What shall we call them? Well, drow is word spelled backwards, so we should do something similar. Fortunately, my favourite word just happens to be RAGREUD! Problem solved. What do you mean that Duergar is unpronounceable? Poppycock! Anyone whose had their jaw broken a few times can say it without issue, and what D&D player out there hasn’t had a few too many fists to the face?”
The perfect monster. If you ignore everything about them.