I've seen the various "ranking" systems from different fictions used to classify a character's ability despite the many flaws with those systems. Just for the sake of getting a list of why they shouldn't be used, I'm go to make this and if you want you can spread it around or stuff. I figure the best place to start are Dragon Ball and the "Big 3" or "HST" as it's called in some places. Be sure to give me any feedback and mention anything I look over.
Ah, good old Power Levels, they can't be wrong can they? It's a simple concept, higher number=stronger character, how could it be wrong? Just look at the Farmer, he's an out of shape man and ranks a 5 and beginning of series Goku who picks up cars, thinks bullets only sting, and has been kicked though stone ranks a 10......WAIT, WHAT? That can't be right, let's try base Roshi he blew up the moon and ranks a 128.....but that's 25.6 the Farmers! I'm sure this fixes it self later like with 1st Freeza who blew up planet Vegeta who has 530,000 and especially with the highest know PL which is Namek saga Ssj Goku with his 1,500,000.........both of which are 10,600 and 30,000,000 Farmers respectively. See the problem? Now I'm not saying "higher number=stronger character" isn't the case since it does work on some level but only when it's taken at just face value and not "X can bust something this many times bigger because his PL is this many times Y's, who busts this". We have no idea what factors into a unit or how much it's worth. Also I'd like to bring up that Ssj and Kaio-ken's multipliers are applied to PL not their stats themselves which an exact multiplier was never stated and stuff like "Ssj level" doesn't make any sense due to them being power ups and have no value on their own.
I'd also like to mention the Kais being "gods" doesn't mean jack. They clearly aren't "omnipotent cosmics" as I've at times claimed. Heck, Kami is "god" but the average Saiyan on Vegeta (like Raditz) could pimpslap the guy's head off and they were "red-shirts". The title of "god" doesn't mean the same thing in everything.
I've seen the "ranks" in Naruto used as tiers which doesn't make any especially considering Naruto ranks lower the Ino yet he could stomp most of his verse. Yes, it's one of the exceptions but still I've never seen in the manga that "if your this strong you're a Genin, that strong you're a Chūnin, etc." and the oddest is "Kage tier". First off, yes they are usually the strongest in the village but that doesn't seem to always be the case especially since we know they're voted in. Look at the 3rd Hokage who was back into the position when the 4th died. The 3rd died trying to stop Orochimaru who has been shown to actually require effort to fight right after post-skip Naruto and Sasuke. One's rank as a ninja doesn't seem to really matter much.
More or less the same as Naruto except replace "Kage" with "Captain". Going by feats, it's pretty clear there's no uniform level of power needed to be one. There's also the Espada and their ranks which for example Ulquiorra makes a joke out of.
I really don't have much to say about OP since fans don't really seem to bring up ranks and the worse I've seen is bounties being used to gauge strength. which is easily pointed wrong from young Robin's 79,000,000 compared to East Blue Luffy's 30,000,000 or the fact Blackbeard didn't have one for the longest time. Bounties only mean how much the world government views them as a threat.