Kael or Athel first. Do you have either of them booked?
We're looking for champs who do as much as possible early when we 6 star them, and the starters do that. They'll be good in campaign farming, Dragon and Clan Boss. I prefer Athel, personally. If you have one of them booked, go with that one.
We need to stop taking several champions to 5*, and focus on taking a few to 6. Dark Athel is one you can stop building out for sure, as she doesn't do too much.