Tier 1: Elune(?), Void Lords, Titans/Pantheon, Sargeras.
Tier 2: Old Gods.
Tier 3: Azshara, Archimonde, Kil'Jaeden, Deathwing, the most powerful Wild Gods, Thunder King, Lich King(?).
Tier 4: Dragon Aspects, Elemental Lords, Titan Keepers, Xavius.
Tier 5: Most Wild Gods, Mannoroth, very powerful mortals like Gul'dan in Legion.
Tier 6: Legion PCs (Highlord/Archdruid etc), warlords of Draenor, other powerful mortals.
Tier 8: Fighters with great skill and/or power, but not as much as tier 6. Includes some class hall champions.
Tier 9: Normal mortals, lesser elementals, lesser demons, etc.