Here is Power Ranking

Low-Class: Low, Mid, High

Mid-Class: Low, Mid, High

High-Class: Low, Mid, High

Ultimate-Class: Low, Mid, High

Satan-Class: Low, Mid, High

Super-Class: Low, Mid, High

Transcendent: the only Transcendent being is Great Red, Prime Ophis, Trihexa, and Yukari with Her Boundary Manipulation

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Yukari Power: Ultimate-Class (High) when she only uses the Physical ability of Youkai, Satan-Class (Mid) when She uses Onmyoudou and Senjutsu, Super-Class (High) when she uses her basic Boundary Manipulation (physical, material, spiritual), Transcendent when she uses high-level Boundary Manipulation (existential, pataphysical, logical, conceptual, metaphysical)

Vali Power: High-Class (Mid) only Magic and Sacred Gear, Ultimate-Class (Mid) when He uses Balance Breaker, Satan-Class (High) when He uses Juggernaut Drive.

Azazel Power: Satan-Class (Mid) with Magic, Satan-Class (High) with Fafnir Armor

Sirzechs Power: Super-Class (Mid) Base Form, Super-Class (High) Destruction Form

Ajuka Power: Super-Class (Mid) with Kankara Formula

Serafall and Falbium Power: Satan-Class (High). Serafall can reach Super-Class (Low) with her Trump Card (it's secret for now)

Michael Power: Satan-Class (High)

Rizevim Power: Super-Class (low) but with Sacred Gear Canceller he is a dangerous enemy for sacred gear users even for Longinus User

Grayfia Power: Satan-Class (Mid)

Kokabiel Power: Ultimate-Class (Low)

Rias and Sona Power: Mid-Class (High)

Sairaorg Power: Ultimate-Class (Mid) with Physical Strength, Satan-Class (low) with Regulus Balance Breaker)

Cao Cao Power: Ultimate-Class (High) with his Martial Art and True Longinus, Satan-Class (High) with True Longinus Balance Breaker

Issei Power: Low-Class (Mid) without Balance Breaker, High-Class (low) with Balance Breaker (the only reason Issei can win Vali in s2 is Plot Armor + Dragon Slayer Sword)