Best friends is 最好的朋友 （zuì hǎo de péngyǒu）or 好朋友 （hǎo péngyǒu）in Mandarin Chinese. However there are lots of ways to say “best friends” in Chinese depending on local speaking custom. Let us take a few examples:

好朋友（hǎo péngyǒu）：best friend
死党：sworn followers, diehard followers
发小：a childhood friend, innocent playmates
闺蜜：a woman’s best female friend, a woman’s best girlfriend
哥们: a man’s best male friend.

sǐdǎng
1. 死党

“死党 (sǐdǎng) ” is something like death party, however it means a very good and close friend in Chinese, “死党 （sǐdǎng) ” is willing to accompany you to enjoy happy,and also willing to accompany you through the pain，they are happy to do anything to help you and share almost everything. If you do something wrong, they may not comfor you, instead they will scold you, as he (she) knows you better than anyone. “死党 (sǐdǎng)” can be between a male or a female.

For example：

Wáng hóng shì wǒ de sǐdǎng.
王红 是 我的 死党。
Wang Hong is my best friend.

fàxiǎo
2. 发小

“发小 (fàxiǎo) ” is a childhood friend, meaning you known someone when you were a child and grown up together , and kept in close touch with each other after growing up. Now many working people bear a lot of pressure from work and complicated relationship and like to spend time with 发小 (fàxiǎo) outside the working hours, they can be true to themselves to stay with 发小 (fàxiǎo).

For example：

Lǐtāo shì wǒ de fàxiǎo, xiǎoshíhòu wǒmen yīqǐ shàngxué, yīqǐ wánshuǎ.
李涛是我的发小，小时候我们一起上学，一起玩耍。
Li Tao is my childhood friend. We went to school and played together in childhood.

guī mì
3. 闺蜜

“闺密 （guī mì）” is a woman’s best/close girlfriend. In an ancient history of China, “闺 (guī)”was the living room where ummarried girl lived. “蜜” literal meaning “honey”. The combination is a woman’s close girlfriend. “闺蜜”usually share their private topics and secrets each other, talk about their feelings and opinions in aspects like love, marriage, working life, man etc.

For example：

我的闺蜜在我过生日送我一个口红。
My best friend gave me a lipstick on my birthday.

gē men
4. 哥们

哥们 （gē men）is a man’s best male friend. “哥 (gē) ”means brothers, it means man regard them as a brother. Young man like to use the word “哥们(gēmenr)”in daily life.

For example:

Gēmen, yīkuài qù chī kǎoyā ba.
哥们，一块去吃烤鸭吧。
Hi brother, let’s go to have a roast duck together.

Related phrases for best friend in writing Chinese:

知己[zhī jǐ]
伙伴[huǒ bàn]
密友[mì yǒu]
挚友[zhì yǒu]
老伙计[lǎo huǒjì]
兄弟[xiōngdì]