Formal:
* Bom apetite! (This is the most common and polite way to say it.)
Informal:
* Apetite! (A shorter, more casual version.)
* Boa refeição! (Means "good meal!" and is also informal.)
Other options:
* Proveito! (Means "enjoy!" and is a more informal way to say "good appetite.")
It's important to choose the appropriate phrase depending on your relationship with the person you're talking to.