I like both of the blue 3*s and have 2 of each leveled up. In terms of damage Gato is better than Valen, but in terms of status effects I like Valen’s fast defense down more. Their synergy is great in a blue stack; Valen -> Gato fired in that order KOs pretty much any enemy in the rarity, and heavily damages bosses of Rare challenge events.
Melia is awesome for her tile damage and crit boost, but she’s mostly a support hero in that sense to me. Bane’s special skill damage and blindness is much better to have. I still use both myself in a yellow stack.
Mnesseus and Berden are both interchangeable. I use 2 of each in a mono green stack with Brienne supporting them. Mnesseus has slightly higher tile damage, while Berden has slightly higher skill damage. Hisan has slightly lower skill damage but he has much better tile damage than the other two, so overall I think he’s the best one of them.
Namahage is easily better than Nashgar, there’s no contest there. Higher attack stat, harder hitting skill, and self-attack boost that becomes very destructive quickly. Sq. Wabbit is on par with Namahage IMO – harder hitting skill without attack boost – while Rudolph is slightly worse than those two, as his skill isn’t as damaging as Namahage’s or Sq. Wabbit’s.
Gill-Ra is purely support, so comparing her to Balthazar isn’t really productive. She’s valuable to have since she’s the only 3* cleanser, and her defense down is great in a purple stack for the Teltoc challenge event. But just having 1 of her is enough imo, whereas duplicates of Balthazar are good to have. You didn’t mention him, but I think Chochin is good too as he’s the only average speed purple hero that hits three for fairly decent damage, along with lowering mana generation. That again synergizes well with Gill-Ra, and can be particularly useful in challenge events against three bosses.