Formal:
* Ciao zio - This is the most common and neutral way to greet your uncle.
* Buongiorno zio - Good morning uncle.
* Buonasera zio - Good evening uncle.
Informal:
* Ciao zio - This is also the most common way to greet your uncle informally.
* Ehi zio - Hey uncle.
* Salve zio - Hello uncle (slightly more formal than "Ciao")
Other options:
* Zio, come stai? - Uncle, how are you?
* Come va, zio? - How's it going, uncle?
Remember:
* If you are unsure, it's always best to err on the side of formality.
* You can also use the uncle's name in the greeting, for example, "Ciao Zio Mario."
Ultimately, the best way to greet your uncle is to use the greeting that feels most natural to you and your relationship.