Since you don't have Spica CC'ed, I would go with Iris. That initial small heal after deployment will help on some maps when you do not have enough healers on the field. After, I would aim to get Spica to max CC level and affection and AW her next. Katie and Bernice would be good choices too as Katie's AW passive gives global 2% def increase and Bernice's AW passive gives 5% def increase to all heavy armors.

In short, list goes like this: G.Iris > Spica > Katie = Bernice > Imelia > Yuyu > anyone you like.