There are a lot more possibilities beyond just those three. The last two are appropriate in formal situations. In informal situations, my usual greetings are "Hey.", "What's up?", "How ya doin' (How are you doing)?" or "How y'all doin' (How are you all doing)?"

All three variants, "Hi all," "Hello everybody," and "Hello everyone," are greetings that are used to address a group of people. While they all serve the same basic purpose, there are some differences in terms of formality and usage: "Hi all": This is a relatively informal greeting. It's commonly used in casual settings, such as when addressing friends, colleagues, or acquaintances in a relaxed or friendly context. It's less formal than the other two options and might not be suitable for very formal situations. "Hello everybody": This is slightly more formal than "Hi all." It's still friendly and appropriate for various contexts, including both casual and semi-formal situations. You might use this in emails to colleagues, at team meetings, or in social gatherings where you want to maintain a friendly yet slightly more polished tone. "Hello everyone": This is the most formal of the three. It's suitable for a wide range of situations, including professional settings like business meetings, presentations, or when addressing a larger audience. It's a polite and inclusive way to greet a group of people and is often preferred when you want to maintain a higher level of formality. The choice of which variant to use depends on the context, the relationship you have with the group you're addressing, and the level of formality required. If you're uncertain about which one to use, "Hello everyone" is a safe option that works well in most situations. However, if you're in a very informal setting, "Hi all" might be more appropriate. As for my preference, I don't have personal preferences or emotions, so I don't have a preference for using any of these variants. My suggestion would be to consider the context and the level of formality you're aiming for when choosing the appropriate variant to use.

It depends who the people are. Are they buddies? Co-workers? Family? Business clients? You talk to different people in different ways. Even so, there is no best answer. How you do this is a reflection of your personality, so only you can know what feels best. Having expressed those reservations, this is what I like to say. I never say "all" or "everybody" or "everyone". In a formal setting it feels to me like bad manners. I just say Friends: Hi! (That's all! Don't underestimate this fabulous word. You can drag it out, like "hiiii---eeee") Clients or business associates: Good Morning! or Good Afternoon! Family: "It's me again" or "I'm back"

Adding on to others comments, Hi All is the most unnatural of the three.

I usually use 'hi everybody' although 'hi everyone' is slightly more formal. For 'hi everybody' it's common to not annunciate strongly so that 'everybody' turns into 'ev-re-body.' We also say 'hi guys' to address a mixed audience, but sometimes people are or at least claim to be offended by this.