Tier 10:0
Regular humans,animals and other creature with no magical power
Tier 9: 1-99
Anyone from the weakest magic user to just below the average Low class devil. Can apply to magic user who have just started their training
Tier 8: 100-199
The varying power degrees of a normal Low-class magic user ,devil,angel ,fallen angel ,etc also applies to most magical creatures.
Tier 7: 200-299
The stages between Low and middle class magical power
Tier 6:300-399
Beings with middle class magical power
Tier 5:400-499
The stages between middle class and high class magical power
Tier4:500-999
High class magical power range
Tier3:1,000-9,999
The stages between high class and ultimate class magical power
Tier 2:10,000-99,999
Ultimate class magical power range
Tier 1: 100000-999999
Satan/Seraph/gov- general class magical power
Tier alpha :1,000,000-9,999,999
Gods,super devils certain monstrous gods, various lovecraftian creature and the heavenly dragons
Tier omega : 10,000,000- near Infinty 
The amount of power reserved for those who are above normal gods and can bring about the end of the planet
Tier : infinity
Infinite power only accessible to the universe  ones. Anyone with this power will be near unstoppable unless is the person who has the same power of level or beyond
Examples: Hades, Poseidon , zeus, great red,trihexa and issei (After he mastered Avalon's full power )

Tier : Multiversal
Beings that Have insane power more then the infinity tier only 1 being is in this realm
Avalon

Some examples rias and akeno will be around Tier 4

Top 10 power rankings
No 0. Avalon Primordial Dragon God of Beginnings the Strongest being In the Multiverse
No 1. great red ,Trihexa,hades,Poseidon,Zeus Issei ( After Master Avalon's Full power)
No 2 . Issei(start), ophis, shiva
No 3 . Eloheim the Bibilical God, Xun er
No 4 . Ddraig,Albion (both before being Sealed ) Valeria, Crom Cruach, Azazel,Nyaruko
No 5. Odin,Thor, Lucina,Sena ,Moka, Xun er,
No 6. Shitzem Livan Lucifer, sayuri Lucifer, Serafall Leviathan, Ajuka Beelzebub
No 7. Loki
No 8. Indra
No 9. Vritra, Fafnir, (Before they are sealed) Tiamat
No 10. Fenrir, Aria