How to say “Come back!” in Japanese?

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too many ways to express this one;

impolite:
戻ってくれ！
modotte kure!

帰ってくれ！
kaette kure!

戻って来てくれ！
modottekite kure!

帰って来てくれ！
kaettekite kure!

modotte — ~te form of modoru, to return, to come back
kaette — ~te form of kaeru, to return, to come back
kite — ~te form of kuru, to arrive, to come, to approch
kure — masu-stem of kureru, to give, to let one have, to be given, to do for one [this is a stem since its all casual speeches] (as you can see, kureru in this case is to tell your target person to do you a favor to come back to you)

to change to polite speech, just replace the kure to kudasai or attach nasai after the ~te after a verb. a commend request verb; ~nasai is a less polite auxiliary verb.