power level

Tier 10: 0

Regular humans, animals, and other creatures with absolutely no magical power.

Tier 9: 1-99

Anyone from the weakest magic user (including but not limited to Issei when he was initially revived) to just below the average low-class devil. Can also apply to sorcerers who've just started their training. 

Tier 8: 100-199

The varying power degrees of a normal low-class sorcerer, devil, angel, fallen angel, etc. Also applies to most magical creatures. 

Tier 7: 200-299

The stage between low and middle class. 

Tier 6: 300-399

Beings with middle-class magical power. 

Tier 5; 400-499

The stage between middle and high class.

Tier 4: 500-999

High-class power. 

Tier 3: 1,000-9,999

The stage between high and ultimate class. 

Tier 2: 10,000-99,999

Ultimate-class. 

Tier 1: 100,000-999,999

Satan/Seraph/Gov. General-like power. Examples: Serafall Leviathan, Azazel, Shimezhai, Michael the Archangel, Gabriel, Metatron. Zach Praktum (Balance Breaker).

Tier α/Alpha: 1,000,000-9,999,999

High level Gods, Super Devils, certain monstrous gods, various Lovecraftian creations that might make appearances in the story much later on, and the Heavenly Dragons. Examples: the Abrahamic God, the original Satans, Rizevim Livan Lucifer, Sirzechs Lucifer, Ajuka Beelzebub, the Leviathan God, Ddraig, Albion.

Tier Ω/Omega: 10,000,000 to ∞ minus 1

The amount of power reserved for those who are above the high gods and can bring about the end of the world. Examples: Ophis, Great Red, Trihexa. 

∞

Infinite power, only accessible to the three Primordial Ones. Anyone with this strength is absolutely impossible to overcome except by someone else with this power. The Only Three: Azrael (Lord of Balance), Satanael (Lord of Order), Kaira (Lady of Chaos).