My first awakens (in order) were Katie, Adele (I was so lucky to roll her early on), Bernice, Spica. I would have done Spica but the big stumbling block for me was farming Sniper Orbs. Awakening the unit I did were a massive help with that.

Adele is nuts with +10% HP to team! If I only have room for one witch its Belinda who gets cut.

If you are aiming at awakening a healer you might as well do spica first/next as their orbs drop from the same map run. Plus Spica makes farming DCs so much better on X map.