Formal:
* Obrigado(a), meu(minha) amigo(a). (Thank you, my friend.) This is the most common and polite way to express gratitude to a friend.
* Agradeço muito, meu(minha) amigo(a). (Thank you very much, my friend.) This conveys a deeper level of appreciation.
Informal:
* Valeu, meu(minha) amigo(a)! (Thanks, my friend!) This is more casual and used among close friends.
* Obrigado(a) pela ajuda, meu(minha) amigo(a). (Thank you for the help, my friend.) This specifically expresses thanks for assistance.
Note:
* You would use "meu" for a male friend and "minha" for a female friend.
* You can also use "amiga" for a female friend, which is a bit more affectionate than "amigo(a)."
Choose the option that best suits the situation and your relationship with the person you're thanking.