Formal:
* 親友 (shin'yū) - This is the most common and formal way to say "best friend." It literally means "close friend" but can be used for someone you consider your best friend.
Informal:
* 親友 (shin'yū) - Can also be used informally, especially among close friends.
* 一番の友達 (ichiban no tomodachi) - This literally means "best friend," but it might sound a bit stiff in casual conversation.
* 大切な友達 (taisetsu na tomodachi) - This means "precious friend" and can be used for someone you value deeply.
* 仲良し (nakayoshi) - This means "close friends" and is a more informal way to refer to your best friend.
Other options:
* ソウルメイト (sōru meito) - This means "soul mate" and can be used for a very close and special friend.
* 相棒 (aibō) - This means "partner" and is often used to refer to a close friend, especially in a male-male context.
The best option for you will depend on your relationship with the person and the tone you want to convey.