Formal:
* Bună ziua (boo-nah dzy-oo-ah) - Good day
* Bună dimineața (boo-nah di-mi-neh-ah-tsah) - Good morning
* Bună seara (boo-nah seh-rah) - Good evening
Informal:
* Salut (sah-loot) - Hello
* Bună (boo-nah) - Hi
Other options:
* Ciao (chow) - This is borrowed from Italian and is also used in informal settings.
* Salutări (sah-loo-tah-ree) - Greetings
The best choice depends on the situation and your relationship with the person you are greeting.