Clusters (highlow retention order):

Cluster 1: S14, S10, S13, S7, S18 (strong early + decent to-end)
Cluster 2: S9, S12, S11, S8, S17 (okay overall; some finish very strong like S9/S11)
Cluster 3 (weak cohort): S19, S6, S16, S15 (low week-one and poor to-end)

I fully expect Season 19 to be ranked as one of the worst seasons, especially given recent dissatisfaction with Stadium mode and lack of meta changing balance changes.