Formal:
* Bom dia (Good day) - Used from morning until noon.
* Boa tarde (Good afternoon) - Used from noon until evening.
* Boa noite (Good evening) - Used from evening until night.
Informal:
* Olá (Hello) - This is the most common and versatile greeting, appropriate for most situations.
* Oi (Hi) - This is a very informal greeting, similar to "hey" in English. It is usually used with friends and family.
Other options:
* E aí? (What's up?) - This is a casual greeting, similar to "how's it going?" in English.
* Tudo bem? (Everything ok?) - This is a polite greeting, meaning "are you well?"
* Como vai? (How are you?) - This is a formal greeting, meaning "how are you doing?"
The best greeting to use will depend on the situation and your relationship with the person you are talking to.