Formal:
* Bonne nuit (pronounced "bonn nwee") - This is the most common and formal way to say good night.
Informal:
* Bonsoir (pronounced "bon swar") - This is a more general greeting used in the evening, but can also be used as a way to say good night.
* Dors bien (pronounced "dor bee-an") - This means "sleep well" and is a more informal way to say good night.
More affectionate:
* Bonne nuit mon/ma chéri(e) (pronounced "bonn nwee mon/ma sheh-ree") - This means "good night my dear" and is a more affectionate way to say good night to someone you are close to.
* Fais de beaux rêves (pronounced "fay de bo rehv") - This means "have nice dreams" and is a sweet way to say good night to someone.
It's important to note that:
* Bonne nuit is always appropriate, regardless of the context.
* Bonsoir is best used when you are greeting someone in the evening, not necessarily saying good night.
* Dors bien and Fais de beaux rêves are more informal and should only be used with people you know well.
Choose the expression that best suits the situation and your relationship with the person you are speaking to.