Formal:
* Good morning/afternoon/evening: Used depending on the time of day, conveying politeness and respect.
* Greetings: A more formal and less common way to say hello, often used in writing.
* How do you do?: A formal greeting used in formal situations, especially when meeting someone for the first time. The expected response is "How do you do?".
* It's a pleasure to meet you: A warm and polite way to greet someone you are meeting for the first time.
Informal:
* Hi: A casual and friendly way to greet someone, suitable for most situations.
* Hey: A more informal and relaxed greeting, often used among friends.
* What's up?: A casual greeting used to inquire about someone's well-being, although not always expecting a detailed response.
* How are you?: A friendly and common greeting, used to show interest in someone's well-being.
* Sup?: A very informal and slang term for "What's up?"
* Yo: A slang term, usually used among young people or friends.
* Howdy: A regional greeting, often used in the Southern US.
* Good day: An informal greeting used in some regions.
Specific contexts:
* Good to see you: Used when you are happy to see someone you haven't seen in a while.
* Long time no see: Used to express that it has been a while since you last saw someone.
* Welcome: Used when someone arrives at a place or event.
* Nice to meet you: Used to express pleasure at meeting someone for the first time.
In addition to the above, you can also use:
* A wave, nod, or smile: These nonverbal cues can also be used to greet someone.
The best way to say hello depends on the context, your relationship with the person, and the formality of the situation.