Formal:
* Bonne nuit: This is the most common and formal way to say "goodnight."
* Bonsoir: This literally means "good evening," but it can also be used as a polite way to say "goodnight."
Informal:
* Dodo: This is a playful and informal way to say "goodnight," similar to saying "nighty-night" in English.
* Bonne nuit à toi: This is a more personal and informal way to say "goodnight" to someone.
Other options:
* Passe une bonne nuit: This means "have a good night."
* Faites de beaux rêves: This means "have sweet dreams."
The best way to say "goodnight" in French will depend on the situation and your relationship with the person you are speaking to.