These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Dragon Quest III
Demonic Spiders: The chest monsters. Man-eating chests can kill your party in one or two attacks when you first meet them, Mimics have a One-Hit Kill spell that will try to kill your party members.
Ensemble Darkhorse: The female soldier. She appears as various NPCs in future installments of the series.
Game Breaker: Depending on when your Mage or Sage gets it, the Transform spell can qualify as one. Having two or more of your best ally? Yes, please! Not much gear though (until the remix throws in quite a handful. The best is the Rubiss Sword, which has a colossal + 160 attack bonus, casts Thordain for free when used as an item, and everyone can use and equip it with no penalties.)