Ralph has the biggest raw damage in-game and the most significant core Max HP.
Felix gives almost 3x more SP from his purple skill than Kristoff’s green.
Belle has similar (high) damage to Ralph, but her skills allow her to stack and increase BD/SP to new limits, and she can use her active regularly, the buffs from red skill and talents are also helpful.
What keeps them from being completely OP, is that their armor/reality isn’t the biggest, but mods can help, and they mainly only do damage. Aurora has much lower damage, but because she removes all buffs and removed a lot of reality, she easily kills enemies.
Megavolt (he is S-tier since the last update) and Ariel are just… much better than before.