There are many variations that English and American speakers use, and each expression means slightly different things. Expressions are also context relevant

#1 Good Morning/afternoon/evening

The earliest known use of the phrase good morning is in the Middle English period (1150—1500). OED’s earliest evidence for good morning is from around 1450, in Sege Melayne. good morning is formed within English, by compounding. Etymons: good adj., morning n.

I regard this appellation formal, polite, appropriate for most circumstances, even the mark of respect

We can say ‘good afternoon’ in an ironic or sarcastic way when someone arrives late for work.  Its a variation on ‘what time do you think this is then?’

NB  Good NIGHT is never a greeting. You are closing the proceedings, even if you meet people at night.

#2 Howdy

Howdy is is a very informal abbreviation of ” how do you do? ” which is common in certain parts of Canada and the US and does not quite fit in with the English way of speaking which might not be taken seriously

#3 Hija

This word is short for how are you and is commonly used in certain parts of England and does not necessarily require an answer. You can just repeat the greeting or say something like “hey !”

#4 How are you doing / how’s everything?

This is an appellation that does not require an answer . I don’t think the average inquirer wants a full medical breakdown of your condition, your failing marriage, your financial circumstances. You could say I’m feeling fine or fairly fine . I noticed that all American cousins say fine unless they’re technically dead at if they are on the borders of death they say fairly fine .

One of the way of losing friends is to say to them, do you really want to know ? And then proceed with your life history. You could give a very brief and it does something that’s happened to you recently, for example, ‘ you’ll never guess who I met’. Do not burden others with your own troubles otherwise people will not ask again and indeed might avoid you.

If you want to answer you can ask them how they are. I think the reply ‘what about you?’ is a bit too interrogative or even menacing. I would rather suggest ‘and yourself?’

#5 Hello

The word “hello” dates back to the 19th century and has its roots in the English language. It is believed to have been derived from the Old English word “hǽlan,” which means “to hail” or “to salute.” In the past, “hello” was primarily used as an exclamation or greeting on its own, but it has evolved over time to become a complete phrase. However there is a good article on the origins of Hello. http://symbolgenie.com/origin-of-word-hello/

The word is also used when you are lost and want to get someone’s attention. ” hello, is anybody there?” it can also be used sarcastically when someone is not paying attention to you. You can say ” hello, is anyone at home?”

‘Hi’ is a shortened and more casualised version of hello.  ‘Hi there’ will work instead but puts a certain distancing in place.

#6 Dear….

When writing letters it is common to write ‘Dear…’ you are not required to know them or have an endearing relationship but simply a personal way of addressing a stranger. You would use it with people that you have professional relationships with

#7 My dear/est ..

This is only to be used with people and is an affectionate intimate greeting. The fact that you don’t have to preface it with anything is indicative of the closeness of the relationship. It can be used in an Oscar Wilde theatrical setting to impress everyone. How are YOU, my dear, or How (the hell) are you or how are you, my DEAREST

#8 Dear sir or madam

This is when you are writing to someone who’s name you do not know, or a missive directive towards a general audience and can be used with formal inquiries or job applications

#9 To whom it may concern

This is for the receiver and probably more in use with business material when you don’t have an example in mind of who you want to receive it. This greeting does not have a personal touch

#10 Yo!

This is more common in the United States though it is spreading across the pond and comes from the 1990s hip-hop slang and it is used both seriously and jokingly. They should be only used with very close friends and never in the business setting

#11 Hi Buddy (budd)

Hi on its own can sound a bit empty so why not hello friend or hi friend or hi buddy. The first word is a greeting and the second word is how you categorize the person. You could say Hi friend but that sounds a bit artificial in which case you would say ‘hello friend’

#12 Good to see you

This can be used in the form ” my goodness it’s good to see you” when you are in a scrape and the person represents a savior or helper in times of difficulty. It can also be used with anyone, an old friend or family member, when it is a relief to see that someone is fit and well after a long period of absence

#13 Nice to see you

The phrase has been commandeered by the English comedian Bruce Forsyth. One of his catchphrases was ‘nice to see you, nice’ so it is rather tainted as a naff expression. It can be used with people that you haven’t seen for some time accompanied probably by a hug or a handshake.

I always liked Bruce Forsyth because his humor was clean, a bit like Tony Hancock and Tommy Cooper. Bruce does Innuendo more than some but he was a family entertainer and he never forgot this.

No one will beat Tommy Cooper’s record of performing in every Hall in the UK but I suspect that Bruce would come fairly close to it certainly in terms of seaside piers though this era seems to have passed, what with the turning of the Theatre on the pier into amusement arcades, which in my view are anything but amusing but then I would say that wouldn’t I.

#14 Long time no see

This is a term of affection especially when you meet someone unexpectedly and particularly after a long period of time. The periodicity is determined by your normal meeting habits so if you be once a week, and you miss a couple of weeks, that would be a long time.  The phrase can be used sarcastically when someone has not turned up for duty

#15 How is / was your day?

This implies a certain degree of continuity and knowledge of the person. If you did not know the person for example if they were a customer in your shop, it would seem more meaningless, sort of robotic mode.

#16 Mate – It’s not what you say it’s the way you say it

In Australia, everyone is ‘mate’ but it can be said in a friendly or threatening way.

#17 How’s it going?

I use this quite often when I go into a shop and it is my way of saying that I don’t have any particular need apart from buying a particular product or inquiring about one. In other words that I’m on point and not coming in the store to do a rant or complaint about something.
I would also use it in a more focused way in connection with say a project that we were both working on. For example if I was doing a garden contract with a colleague and had to absent myself for some reason, on my return I would use this phrase.

#18 Are you OK?

This is hardly a greeting more a context sensitive question. You could have stepped out of an accident site and looking a bit shaky on your feet or you can have had a recent occupation or illness and you are on the mend so the form of inquiry is meaningful and directed not just a general inquiry. You would use this term if someone looked ashen faced for no reason and no context

#19 Greetings!

I see this as more ceremonial and formal when you greet someone to a place or a special occasion and it is not just you doing the greetings, it is greetings on behalf of other people who may be organizing or participating in an event. ‘Greeting’ a complete stranger seems to me more devoid of meaning or significance.

#20 it is a pleasure to meet you

Or sometimes ‘its nice to meet you’ This is normally when you have heard of someone and they have a good reputation, or it is someone with whom you have spoken on the telephone and have never actually met in the flesh.

#21 – How are you?

See #4 above. Be careful asking this one. You might be lucky and get away with someone who says ‘ I’m fine’ but you might get a life history. Do you have time for this? Most people do not. It is safer to say yes thank you and you? They will then confirm or otherwise their fineness. If someone says ‘ I’m good’ I say that I’m not interested in their moral  state, just there emotional state.
How have you been?  This means ‘how have things been generally’ or ‘during your recent illness have you been able to perform well?’ or ‘what has gone on recently?’

#22 – What’s Up?

This can be very easily confused with the recent arrival of WhatsApp? It could be seen a shorthand for ‘ as anything happens since we last met’. In which case the answer would be ‘ all good’. If it was not, then you would give details of the event

#23 – what have you been up to (then)?

This can be the mildest form of chastisement or teasing particularly of someone who is known to have an adventurous lifestyle. Let’s say your friends has been away with someone of the opposite sex you would want to know or would be curious to know some details but it can also simply be the basis for bantering and humor

#24 – Hey!

This is not so much a greeting, more a device to get someone’s  attention. I think you have to be careful how you use your voice here as it can seem commanding or kajoling.