A reliable tier list should not only sort by rarity. Base DPS, cooldown, range, cost, passives, skill hits and gear synergy all change how a unit performs.
For the first release, use a transparent data score: DPS, DPS per cost, range and defensive value. Mark it as a data-based ranking until more stage wave samples are collected.
Secret and evolved forms tend to dominate late-game scoring, but lower-rarity units can still matter for early Story progression because of cost and cooldown.