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.
Game Breaker: Lloyd and Randy's Combo Craft, Burning Rage. Especially the powered-up Burning Rage II. Better yet, Burning Rage III.
Tio's Zero Field, if activated when CP gauge is full, shields all party members from two attacks. There's a recipe that can refill 95 CP. You get the idea.
In Ao no Kiseki, Wazy Hemisphere hands down, once he rejoins and you get access to his S-Craft Akashic Arm. Easily one shots any boss you come across, and if they're still standing, you need very little in the way of preparing another shot to knock them down hard. All you need to watch is this. It doesn't help that there's a recipe which does two things: refills CP up to 100/200, or take your HP down to 1. And both benefits Wazy, once you figure out the right setup (which takes little effort, except for the winning all POM games).
It's possible to make Burst lasts forever by using Burst Orbs during Burst. Burst Orbs are farm-able. Do the math. Explanation - When in Burst, enemies won't attack your party. So it's possible to win any battle without them attacking you. And that's including the Final Boss, if you can execute Zero Field at the right time.