Formal:
* 晚安 (wǎn ān) - This is the most common and formal way to say goodnight. It literally means "peaceful night".
* 祝您晚安 (zhù nín wǎn ān) - This is a more polite and formal way to say goodnight, meaning "I wish you a peaceful night".
Informal:
* 晚上好 (wǎn shàng hǎo) - This literally means "good evening" and is a more general greeting used later in the day. It can be used as a casual "goodnight".
* 睡了 (shuì le) - This literally means "I'm going to sleep" and is a very casual way to say goodnight.
* 明天见 (míng tiān jiàn) - This literally means "see you tomorrow" and is a casual way to say goodnight, implying you'll see them again soon.
The best option will depend on who you are talking to and the situation.