Here are a few options:
Formal:
* 挚友 (zhì yǒu): This literally means "cherished friend" and implies a deep, long-lasting friendship.
* 知己 (zhī jǐ): This means "someone who knows you well" and implies a close friend who understands you deeply.
Informal:
* 死党 (sǐ dǎng): This literally means "death party", but it's used in a playful way to refer to a close friend who is like family.
* 好兄弟 (hǎo xiōng dì): This literally means "good brothers" and is used for male friends.
* 好姐妹 (hǎo jiě mèi): This literally means "good sisters" and is used for female friends.
The best way to choose the right word is to consider the relationship between the two people and the context of the conversation.
It's also important to note that Mandarin speakers often use phrases to express the concept of "best friend", such as:
* 最好的朋友 (zuì hǎo de péng you): literally means "best friend"
* 最亲密的朋友 (zuì qīn mì de péng you): literally means "closest friend"
Ultimately, the best way to convey the meaning of "best friend" in Mandarin is to use a combination of words and phrases that reflect the specific nuances of the relationship.