Steel Iron rank- Consists of E-ranks and F-ranks. Guild leader can be any of the E-ranks
Dark Bronze- Guild leader is D-rank
White Silver- Guild leader is one C-rank or three and above D-ranks
Cold Mithril- Guild leader is B-rank
Black Gold - Guild leader is one A-rank
Supreme Diamond- Guild leader(s) is either one S-rank or three and above A-ranks.
Equipment, consumables and artifacts follow tier and rarity ranking. Tier-0 is F-rank and tier-1 is E-rank, and so on. Rarity is the quality of items within their tiers. In some cases a lower tiered item is far more priceless than a higher tiered item purely because of rarity
Common
Rare
Epic
Legendary
Mythical
[Unique]
Unique items are a special case. Under normal circumstances Mythical is supposed to be the highest however Unique rarity means unlimited potential. Even if a unique gun is initially weaker than a tier-0 sword, given enough time and hard work, it would effortlessly destroy tier-3 weapons while at tier-1 if raised right.
These rarities apply to anything including classes, just note that special classes and inherited classes are usually legendary or mythical. Evolved classes are rare or epic. Normal classes are common.