Formal:
* 晚安 (wǎn ān) - This is the most common and neutral way to say "goodnight." It literally means "good evening" but is used as a polite way to say "goodnight."
* 祝您晚安 (zhù nín wǎn ān) - This is a more formal way to say "goodnight," literally meaning "I wish you a good evening." You would use this when addressing someone you are showing respect to, like an elder or a teacher.
Informal:
* 睡了 (shuì le) - This literally means "slept," but it's used as a casual way to say "goodnight" to friends or family.
* 早点休息 (zǎo diǎn xiū xi) - This means "rest early," and is a good way to say "goodnight" to someone who has been working hard.
* 好梦 (hǎo mèng) - This means "good dreams" and is a sweet and friendly way to say "goodnight."
You can also add a "night" at the end of any of these phrases, such as "晚安，晚上 (wǎn ān, wǎn shàng)" for "goodnight, evening."
The best way to choose the right phrase is to consider the situation and your relationship with the person you are talking to.