Shonen anime love to quantify the strength of a fighter by using some sort of numerical power system. The forefather of this idea is the Battle Power of Dragonball. If your number is bigger than the other guy’s number, you win. It’s very straightforward and eventually meaningless, since anyone can suppress his ki, or power up hundreds of times with various means, thus meaning nothing beyond telling you how strong is someone with a number, without having to prove it onscreen.
This is a bit improved as a concept in the Reiatsu of Bleach. It doesn’t use numbers in-series and relies in ranking the fighters in their base form, with two digits at the most. The smaller the number of a division captain, or an Espada, the stronger he is. It still doesn’t mean much, since everyone can become hundreds of times stronger by releasing their damn bankai goddammit, but it’s definitely an improvement over the arbitrary numbers of Dragonball that can go up to septillion in digits and still mean nothing.
The Power Levels of the Seven Deadly Sins improves this concept by making the number a sum of 3 subcategories. You are ranked in the military based on the sum of your base form, just like in Bleach, but your fighting skills are a bit more specialized. It’s still arbitrary since everyone powers up like crazy, but on paper someone with a lower power level can defeat you if his subcategory is much higher than yours, offering much more variety in battles.
The Elemental System of Avatar improves this by having four different elements, each one with its own style and philosophy. It’s not just a number; it’s also a way of life. And yes, I know it’s a cartoon, shut up.
The Chakra system of Naruto improves this even further, by adding elements that can be effective against one and weaker against another. The conversion cost is also interesting, as you have a total sum of life energy you can’t fully tap into without wasting some of it during its conversion into elemental magic, martial arts, or illusions. Yes, it’s treated like a joke once eye techniques and chakra monsters overpower everything else, but on paper this is a much better power system.
As you noticed, none of the systems is used with consistency, since shonen are notorious for setting up a lot of rules and then break them all for the sake of spectacle. Light novels and porn games take this to the extreme by spending hours in elaborate scientific explanations and then have the protagonist being special and able to break every single one of those rules with an ability only he possesses. It’s a necessary evil most shonen are doing, since the protagonists have to be special, and thus cool.
Fate Stay Night: Rule Breaker > rest
Index: Magic nullifier > rest
Dragonball: Saiyans > rest
Bleach: Ichigo half of Everything > rest
Avatar: Mastery of all 4 elements > rest
Naruto: Six Paths & Sharingan > rest
Even so, the damage multiplies when they attempt to use more than one system at the same time, without bothering to explain how they work. RWBY for example has three different power systems. Aura used by everyone, magic used by the four maidens, and silver eyes used for freezing monsters. They were never defined properly and since everyone is as strong as the plot wants him to be, it comes down to random nonsense.
One Piece does a somewhat better job by treating every system as its own thing. The Bounties are a way to rank the pirates by notoriety and danger level, which can be completely irreverent to how strong they actually are. A simple way to define strength initially was the three types of Devil Fruits, which had a clear escalation of power. Then martial arts and Haki were introduced, both of which are completely different systems that can be used even by non-devil fruit users. And yes, even that show is not that good in execution, since the good guys have infinite stamina, so it doesn’t matter how strong they are in the longrun. All I can say is that at least this way everyone can have a fighting chance by using a power against someone who doesn’t possess it. Instead of constantly increasing arbitrary numbers, which inflate their purpose and result to power creep, this comes down to using a technique the other one can’t defend against, regardless of how powerful he is. It makes power ups non-linear and helps minor characters to remain relevant even late in the story by focusing on something the main characters cannot do.
I might as well mention the Nen types of Hunter X Hunter, which don’t work in a rock-paper-scissors style, like the elements do, and unlike the powers in One Piece you can master only a few of them at best. They are completely isolated from each other, none is inherently overpowered and it comes down to how you use them instead of how powerful they are against other types. They were also ruined once magical wishes became a thing that break all the rules, but besides that this is the best system to date.
