Titans are king
Okay.
Titan loot tiers
My alliance is still fighting 6* titans after 2 years.
Triple bosses guard most my 4* ascension items and emblems.
What is your roster
I am leveling up my Little John ( slow mana, debuff all enemies), but for other players it is less helpful:
So I would personally level Miki, but for players with Hel, Tarlak and Ranvir, they very likely have better options.
Versatility, Flexibility, Utility
I would love if costumes made every hero useful in multiple situations, but that decision is up to the design team.