Someone on 4ch posted a list like this, so I figure it might be good to include it for those wondering what units are good to get at the start. This has been modified a bit to suit my own opinions on some units.
For rerolling:
Great gold units:
Bernice, Themis, Iris, Mehlis.
Good gold units:
Julian, Barbastroff, Cyrus.
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Great plat units:
Gellius, Garania, Thetis, Lilia, Sherry, Bashira, Camilla, Victoria
Good plat units:
Cellia, Dahlia, Jerome, Lyla, Sakuya, Hina
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Great black units:
Sybilla, Deine, Nanaly, Despara
Good black units:
Anything else
When re-rolling accounts, remember that you can continue using the same e-mail and user name by following this link and closing the account:
https://www.nutaku.net/login/?url=/profile/delete/