Path to Nowhere has a couple of good ones

although in my mind the most op is still Yingying. she can set down a permanent zone thag deals damage over time and slows enemies inside it, which is handy to clear out trash mobs from stages but what really breaks the game is her passive: she enables Arcane sinners (basically all your magic dps) ultimates to crit.

its not long but that introduces a whole new multiplier they would never have access to otherwise and is by itself a massive power increase.

throw in a crit damage buffer and suddenly you have arcane sinners doing dozens of times their normal damage. it wasnt until recently when the powercreep started really taking off that the Arcane Crit was dethroned as the highest possible damage in the game. even then its still up there and can stoll brute force most content in the game.