Formal:
* Bonne nuit, mon amour. (Good night, my love.) This is the most common and straightforward translation.
Informal:
* Bonne nuit, mon chéri(e). (Good night, my darling.) This is more intimate and affectionate.
* Dors bien, mon amour. (Sleep well, my love.) This is a sweet and caring way to say good night.
* Passe une bonne nuit, mon cœur. (Have a good night, my heart.) This is a very romantic and poetic way to say good night.
Even More Intimate:
* Bonne nuit, mon/ma tout(e). (Good night, my everything.) This is a very tender and deeply affectionate phrase.
The best way to translate "good night, my love" in French depends on the context and your relationship with the person you are speaking to.