Formal:
* Greetings: Good morning, good afternoon, good evening
* Hello: This is slightly more formal than "hi"
* How do you do? (Though this is more of a question than a greeting)
Informal:
* Hey: More casual than "hi"
* Yo: Very informal
* What's up?
* Howdy: (Southern US)
* Sup? (Short for "what's up?")
Other options:
* Good to see you: For a more personal touch
* Nice to meet you: If you're meeting someone for the first time
* Long time no see: For a friend you haven't seen in a while
The best word to use will depend on the situation and your relationship with the person you're greeting.