All tiers always give a descending amount of League Credits starting from #1; however, all chests have the same possible contents; ranking highly only gives better chances for better drops, and may guarantee higher level gear drops (it is uncertain how this plays out across different raid difficulties).
It appears ranking #11-15, #16-20, and #21-30 results in the same raid chest, and they only differ in the amount of League Credits.
Raid IV[]
The follow images is from Raid IV, as indicated by the 4 out of 6 diamonds in the description for the chests.
