In no specific order, I would put priority into leveling these units and fitting them into your squad when applicable and your cost allows it. Remember, it's usually better to have a full team of 6* units compared to a 7* unit and four 5* unit party due to cost restrictions.
Lunaris (5*)
Kafka (6*)
Edea (6*)
Ruby (5*)
Lucia (6*)
Zelnite (4*)
Tesla (5*)
I would put priority on leveling this units. Lunaris provides Status cleanse and protection for 3 turns. Kafka is awesome for inflicting status ailments on your enemies. Edea goes from being supbar at 6* to an amazing mitigator unit in her 7* form. Ruby is an all around great unit as a lead for BB fill and has a very powerful UBB that allows your party to deal insane amound of normal attack damage. Lucia (the free unit we got from the anniversary) is your only unit that buffs critical hits so that will be handy. Zelnite's 6* form is great for his leader skill for item farming in quests. Tesla (Zeldeus in its 7* form) is another mitigator to add to your arsenal.
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