1) "Hey there": This is casual, friendly, and familiar. It could be used between good friends or romantic partners in spoken English, text messages, voice mail messages or emails.
2) "What's going on?": This is casual and could be used between friends or acquaintances in an informal situation like a party.
3) "Hey! What's up?": This is casual and could be used between friends, colleagues, siblings, etc.
4) "Good morning" or "good afternoon": These greetings are generally more formal and are often used in restaurants, hotels, shops or at work between colleagues.
5) "How are you doing today?": This is formal and might be used between a professional and a client or customer or between colleagues or a boss and his or her workers.