Generally speaking, for combat-relevant tiers, it goes like:'
- Low-class Devil level: Should include people such as Rias' peerage (save for Akeno) prior to training for the Riser Rating Game and most of Riser's Pawns.
- Mid-class Devil level: Not sure of which characters actually fall at this tier, but if I had to guess judging by how Yuuto with his Balance Breaker could "easily" defeat a Mid-class Fallen Angel thanks to its abilities, I would say pre-BB Yuuto, base Koneko before Volume 5 and Volume 3 Irina and Xenovia (without Durandal) would be around this level.
- High-class Devil level: Includes people like Riser, Issei with his incomplete Balance Breaker and initial Balance Breaker state, along with base Sairaorg.
- Ultimate-class Devil level: Has pre-Azazel Cup Cardinal Crimson Issei and pre-Azazel Cup Balance Breaker Sairaorg. High ends of this class could be in a subclass for "Dragon King level", with people such as Bikou, perhaps Grendel and Ladon, and Ryuuta.
- Maou-class Devil level: This one also has respective sub-classes, as it can be used to refer to people comparable to the former Four Maou (weaker end) or to the second generation, current Four Maou (stronger). People described as Maou-class include current CxC Issei and current BxB Sairaorg, Grayfia (stronger subclass), Serafall (stronger subclass), Diehauser (stronger subclass) and Falbium (stronger subclass). The stronger end of this class can manage defensive battles against gods or can even fight on par with and defeat gods who are not combat-focused.
- "Beyond" Maou-class: A vague tier applied to Balor Rias primarily, and in lesser measure seems to also include Sonneilon and Gressil. Balor Rias could rather easily dispatch a high-end Maou-class like Grayfia, fought an 80% Fenrir without being at a disadvantage (such an opponent should at least be "normal" God-class, and Rias only lost because stamina issues weakened her) and also had the chance of injuring PDxD Issei in the Heavenly Dragon level. However, it is unknown how much of this was covered by Rias' Power of Destruction's unique qualities, hence I am conservatively putting it somewhat below normal god-class (unconfirmed).
- Normal God level: God-class characters begin to have a huge spectrum of power difference despite being all called by "god-class", so I will use this one to refer to characters such as Loki and Artemis. Loki showed clear superiority to characters who should be comparable to the lower end Maou-class when he blocked without issue attacks from both Azazel and Baraqiel, but a higher end Maou-class like Grayfia, with adequate assistance, could manage a more or less equal battle with Artemis. Going by his performance against Xenovia + Irina + Ryuuta, Strada should either be high end in this tier or halfway through to the next one.
- Chief God level: Ex mentioned this class to describe the Seven Luminaries, so I think it's fair to include it. Should refer to people on the level of Apollon, and maybe Odin.
- Lower Top 10 level: We enter the true big leagues. Characters here should include Vidar with his special shoes, and by extension Thor, as well as people such as Typhon and Fenrir via retcon.
- Middle Top 10 level (Heavenly Dragon-class): Includes people like DxD L Vali, PDxD Issei, Ddraig, Crom Cruach, as well as Azi Dahaka and Apophis. Characters in this tier are noticeably stronger than the previous one, but they can still be harmed by the lower end of the Top 10 and fights between them are not absolute stomps, as Boots!Vidar could harm Issei in PDxD and Typhon noticeably tired Ddraig out. People in this tier have an ambiguous relation to the Super Devil tier, with Ddraig describing it as such that they can fight them, but this becomes vague with the immense difference within that tier.
- Super Devil level: One of the more complex tiers. It includes people who are noticeably superior to the high end Maou, but these people can also have immense difference among themselves. Proof of this is in Rizevim, who was much weaker than a Heavenly Dragon-class, and was described as much weaker than Sirzechs and Ajuka, who should be comparable (perhaps weaker or stronger, though likely not by an immense margin). This discrepancy makes this tier hard to judge.
- Middle-High Top 10 level: Includes people such as Indra and Mahabali with his special weapons. Implied to require two Heavenly Dragon-class fighters in a well coordinated tag team to take one of these guys out. Incomplete Ophis, going by her power level being twice of a Heavenly Dragon, might be included here (not confirmed).
- High Top 10 level: Includes people like Shiva, who was the one specifically addressed by Azazel to deal with Trihexa if he got unsealed, hinting that people in this tier might be able to at least do something against it and maybe a Dragon God-class, but according to Cao Cao's words, a great difference still exists between this tier and the Dragon God one.
- Dragon God level: Normally the highest tier. Has people like Ophis and Great Red as its high end and Trihexa as its lower end. A single being in this tier is enough to take on pretty much armies of most of the previous tiers, being able to handle even god-classes by the dozens if the Isolation Barrier Field battle is anything to go by.
- "Evil God" level: Unofficially termed tier for people like Melvazoa and his siblings. This should be the highest tier for now, and people in this tier are noticeably superior to even the high ends of the previous tier going by "Secret Talk".