right now u don't need to skill up anything....but if you were to skill up something , go with your highest rarity units usually....or your most used ones.

That being said , u're better of keeping your rainbows untill skill awakening. Actually if i needed to make some space , without using scs to expand my baracks >>> which you should btw <<< , i would probably CR some of the gold units (bernice/fedora/iris). But you're better of expanding your baracks IMO