Tier 8: 0
Regular humans, animals, and other creatures with absolutely no magical power.
Tier 7: 1-99
Anyone from the weakest magic user to just below the average low-class devil. Can also apply to sorcerers who've just started their training.
Tier 6: 100-299
The varying power degrees of a normal low-class sorcerer, devil, angel, fallen angel, etc. Also applies to most magical creatures. Examples: Raynare, Mittelt, Kalawarner
Tier 5: 300-499
Beings with middle-class magical power. Examples: Akeno Himejima, Tsubaki Shinra
Tier 4: 500-999
High-class power. Examples: Rias Gremory, Sona Sitri, Riser Phenex
Tier 3: 1,000-9,999
Ultimate class. Examples: Kuroka, Sairaorg Bael
Tier 2: 10,000-99,999
Satan-class. Examples: Serafall Leviathan, Grayfia Lucifuge, Yasaka
Tier 1: 100,000-1,999,999
Super-Devil-class. Examples: Sirzechs Lucifer, Ajuka Beelzebub, Rizevim Livan Lucifer, Heavenly Dragons
Tier α: 2,000,000-9,999,999
Dragon-God-class: Examples: Great Red, Ophis
PS. This is just straight out of my ass, this isn't the official power rating of DxD