最好的朋友 (zuì hǎo de péngyou)
* 最好的 (zuì hǎo de) means "best"
* 朋友 (péngyou) means "friend"
You can also use:
死党 (sǐdǎng) - This literally means "dead buddy" and is a more informal and playful term for a very close friend.
知己 (zhī jǐ) - This translates to "soulmate" and is used for someone who you deeply understand and who understands you.
The best option depends on the context and how close you are to the person.