Formal:
* Caro(a) amigo(a): This is the most formal option, literally translating to "dear friend". You would use this for someone you don't know well or are addressing in a professional setting.
Informal:
* Querido(a) amigo(a): This translates to "beloved friend" and is a more affectionate and intimate way to address someone.
* Amigo(a) querido(a): This is similar to the previous option, but puts the emphasis on "friend" first.
* Meu(minha) querido(a) amigo(a): This literally means "my dear friend" and is the most affectionate way to say it. It implies a very close and loving relationship.
Other options:
* Amigo(a) do(a) meu(minha) coração: This translates to "friend of my heart" and is a very poetic and heartfelt way to say it.
* Amigo(a) especial: This means "special friend" and implies a close and valued relationship.
The best way to choose the right phrase depends on the context and your relationship with the person.