I implemented a system in my homebrew campaign that I need help managing

So in my campaign, I've made it a system that most creatures and villains, including basic npcs drop something called a magic core, that can be used to strengthen weapons, armor, or give a temporary boost to a stat. It can also be used to boost the strength of a spell by a certain amount of levels. The stronger the creature, the more powerful the magic core. I'm just trying to find a balance and rank them so I can set the strength of each core. What would be a good ranking system?