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: Breath of Fire I
Game Breaker: The Agni transformation makes the game incredibly easy. Merging the whole party into a single being, max HP, an attack that does 999 damage, and status immunity would do the trick. Have a few Cures (or Melons) and you can beat the game easily.
The (Boomer)Rang weapons. Combine them with Mrbl1 items (a cheap, easily obtained item that guarantees Critical Hits), and you will wipe the floor with the entire enemy group with just Ryu.
For that matter, Mrbl1. Just Mrbl1. These also work on Ryu's dragon attacks too. Carry entire stacks of these around, and bosses die much faster. Well, until you get Agni.....
Karn's fusion forms are similar. He combines with two or three other party members for a major stat boost. He'll always attack first and will deal more damage than anybody except Ryu's dragon forms and Bleu's magic. Unlike Ryu's dragon forms, Karn's fusions remain after the battle, so unless you need a map ability, there's little reason not to have him fused.
White magic always moves first in a battle, unless it is an attack spell like Zom1. Nina becomes completely invaluable in any boss battle as a result.
Funny Moments: In the manga adaptation, Ryu initially met Nina as the latter was taking a bath. He quickly hid himself right when two Dark Dragon minions decide to show themselves. Nina accused them of being the culprit...and they promptly apologized
"Wait! We of the Dark Dragon Clan are already half the world's sworn enemy!!! And we're scared of a woman?! Damn!!! We've been confused with magic again!!"
Seinfeld Is Unfunny: One of the earliest JRPG's to hit America, so it was bound to become this.
The Scrappy: Gobi and Mogu. Both join very late along with Ox, but Ox at least has a backstory and sympathetic character. Gobi just exploits your party for money, then does it again to save Ox, and Mogu has an annoying recruitment level and is generally useless in battle.
Tear Jerker: Cerl and Alan's ultimate end, definitively. In The Dragon Warrior manga adaptation, Jade and Sara's final hopeless stand against Myria counts as well.
The former is subverted, however Cerl and Alan get turned back into children due to the Time Key's effects, more or less giving both of them a second chance at life, and allowing Cerl to remake happy memories with Alan.
That One Boss / Wake-Up Call Boss: The Gremlin. He has a ton of HP and will start spamming a powerful group-hitting attack when his initial health bar runs out (at which point his HP is only about 1/4 to 1/3 of the way gone).
That One Level: Most of the surreal dungeons on Mote's dreamworld, the crystal-path maze with the rotating floor being specially loathed.