It depends on what you need once they hit AW and Skill AW I think (Correct me if I'm wrong)
Waltz - Keeps a good tempo with her constant good stat boost once her skill is on.
Marnie - Crescendos and Decrescendos with skill usage with a side of -1 UC to any units you've yet to place.
Hana - Her range is decent and comes with an emergency heal like if you placed AW Iris or Camilla.