Formal:
* Hola: This is the most common and versatile greeting. It's appropriate for most situations, even when addressing strangers or someone older than you.
Informal:
* Hola: It can also be used informally.
* ¡Hola! (with an exclamation point): This adds emphasis and is more common in casual settings.
* Qué tal: This literally means "How are you?" and is a more informal way to say "hi."
* ¿Qué onda? (literally "What's up?"): This is very informal and only used with close friends or family.
Other Options:
* Buenos días: Good morning.
* Buenas tardes: Good afternoon.
* Buenas noches: Good evening/Good night.
The best choice depends on the specific situation and your relationship with the person you are greeting.