Rigard and Melendor ( together they fight crime)
I believe Rigard plus Melendor is the most powerful pair of 4* healers in the game.
Charge at different rates because different colors ( this is a plus ), both 42% immediate heal, one removes ally debuffs, one removes enemy buffs ( without taking damage)
My 4*+18 Rigard plus 4*+18 Melendor get used constantly for quests, seasonal events, challenge events.
Classic 4* / 5* heroes will be the first to receive costumes. Allowing them to act as healers for additional Class quests.
I am currently raiding with Rigard, Melendor, Frida, Grimm, Valeria.
Special skill control
I love my 4*+1 Hansel for rainbow raiding, but Hansel is not as good as my Proteus ( 4*+15 and 4* 3.60 ) for triple bosses.
Infamous Wu Kong
Wu Kong takes practice, but I believe Wu Kong is the single most powerful 4* hero in the game.
At least until they make a 4* Tarlak ( hint, hint Dev Tim. Maybe a Tarlak costume for Wu Kong ).
Often I will just use Wu Kong for his base stats and hold him with a fully charged mana bar until the enemy are debuffed, my team is buffed, and there are one, or more, power shards on the board.
Famous Proteus
I believe Proteus is the single most useful 4* hero for completing Challenge events ( except reflect purple) allowing me to get emblems, 3* ascension items and world energy flasks from tier completion. So far Proteus is equally as useful for Seasonal events getting me 3* / 4* ascension items and ingredients.
Famous Jackal
I believe Jackal is not as good as Falcon, but even dead ( grave damage) is very powerful for color stacking
(Two Rogues or comparing Guardian Jackal and Scarlett ( with a mention of Grimm ))