Now I won't lie. The power system in DxD is seriously fucked up because of issei. I can never gauge his power accurately. Just know that now he is strong enough to fight and beat gods so for those who already know the classification he surpassed the Satan class and even most super devil or equivalent are inferior to him.

Here the classification

LOW-CLASS:

-low-class devil

-two-winged angel/fallen angels

-low-class exorcist 

MID-CLASS:

-mid-class devil

-four-winged angel/fallen angel

-mid-class exorcist 

HIGH-CLASS: The high class is kind of a limit. Many are able to reach it but few are able to surpass it. For the human, it's close to impossible to reach this class without having a sacred gear or some kind of legendary weapons such as Durandal. The only one who broke through that limit and nearly reached the ultimate class was Vasco Strada. Don't talk about Dulio. Dulio is a living cheat code. Even then the number of high-class exorcist is pitiful and most are only comparable to mid-class. You could also talk about magicians but for me they are a little different from normal humans

-High-class devil

-six-winged angel/fallen angel

-high-class exorcist  

ULTIMATE-CLASS: ultimate class are basically nuclear war weapons. Ultimate classes could destroy entire nation by themselves. So yeah. When you enter this class only then can you really be respected? 

-ultimate class devil

-eight and ten-winged angel/fallen angel

SATAN-CLASS

-Satan-class devil

-Twelve winged angel

SUPER DEVIL CLASS: The super devil class is extremely hard. Like only three Devils reached this class. Even for the Gods, not many reached it

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Now those classes I used are mostly Human, angels and devils with gods appearing toward the end. The fans know of course that this isn't all. There is also the yokai, the vampire and other such races. Finally, there's being who are basically a class themselves such as the dragon. For the yokai, the strongest I know is Yakasa who is a kitsune and is comparable to an Ultimate class. Kuroka should also be close to ultimate class. But not sure. 

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THE DRAGONS

-TRUE DRAGON: There's only one true dragon 

The dragon of dream Great Red. (AN: What a coincidence since I am REDLAW.)

-DRAGON GOD: Only one being is called dragon god and that's Ophis the dragon of infinity, the Orobouros dragon. Some can speak about Lilith but well she is basically Ophis clone so... 

-HEAVENLY DRAGON: There's only two heavenly dragons. The red dragon(again what a coincidence) and the white dragon. Those two are superior to Super Satan class since they're said to be the killer of gods and Satan. In order to beat them the leaders of all the three factions had to fight together. 

-FIVE DRAGON KING: Apart from the Dragon King Tiamat who is comparable to Satan class . The other four are comparable to ultimate class. 

-HIGH-LEVEL DRAGON: Basically the lowest level for an adult dragon. Yep. The lowest level for dragon is a level most being can only hope to reach. 

-LOW-LEVEL DRAGON: Those can't really be called dragon. More like a failure of evolution. They lack the intelligence dragon are known for and are more like giant reptiles than dragons. 

EVIL DRAGON: Those bunches are a group of little bastards with no exact power level. Still, most of them are stronger than Dragon King with The strongest Crom Cruach being said to be equal in power with the two heavenly dragons.

Now there's one thing you must know about dragon in DxD they are basically bug code. They have a mode called [OUTRAGE], it's basically the reverse scale of the dragon for those used to read CN.

Anyway, that mode is like a super booster. They're basically like mc from xianxia story. When in outrage, they can jump many levels and beat down people they should have never been able to beat.

One, for example, went from ultimate class to super devil for a short instant and bitch slapped a major villain.