The only maiden I would consider mandatory is Althea for her heal.

Micah always hits at least one enemy with her skill. Her main thing is her huge attack range and huge skill aoe. Her skill cooldown is also quite fast.

Heet feels kinda bad, but haven't used her in combo with Althea yet. Other than self damage she has problems with her short range and fast running speed getting her into trouble.

Luna is slow, and her skill takes an eternity to trigger.

Nuka is bad. The range on her skill is shorter than her normal attack range. A lot of the time it triggers on a full health ally instead of under the tank & enemy.

Suzume is only good for farming stages. On difficult stages she runs ahead of the tank and dies.

Agni seems kinda pointless.

The skills of Coral & Aria are both damage over time. Stuff generally dies long before they fade.