My best characters are 4 star, level 80, Corrupt Harpy (Purple), Pharaoh Dog(Blue) and Pyro Soldier(Green).
I personally use the CH and PD as my main party, CH hits really hard and PD can heal the whole party and chow down on enemies. PS doesn't hit very hard but he doesn't die easy at all and has a big health pool. Any recommendations on which one I should set to Captain for my troopers? I alternate between CH and PD.

Of course if the RNG gods ever give me a 5 star anything that'll be slotted in as Captain