Whose Potential Should I Unlock Next?

So after getting two beast units (thanks IS, very cool), I realized that I have enough feathers to unlock a unit's potential. I don't know who though since I have way too many choices so I just thought why not have the decision be made between these units. I think I'll be done with summoning from the beasts banner after being done with all of my summon tickets. And thank you for any who participates and/or gives advice :)