I imagine it would be ‘你好‘ 。 there is also 好久不见，你最近怎么样，你好吗. I'm sure there are plenty of other greetings, these are just some I know.

Hi Miguel, I think there might be some culture difference you need to take into your consideration. Chinese are more conservative than people from western country. So in our traditional culture we do greeting each other by using "xxx,你好"，“xxx,早上好” etc. while with the development of economy and culture, more foreigners came to China and more Chinese went abroad, we begin to say "嘿" or “嗨” .As you are just wondering the most common greetings among friends. You could just say "嘿 ” or 嘿plus anything you wanna ask or talk. in fact it would be weird if you say"你好" among friends. today "你好" is often used in formal occasions or the first time you meet someone. So, just feel free. you need to remember that we have different culture:)

嘿~ ==> Hey (Hi works too. It's just random stuff like that which transcends languages, especially with everyone watching American TV shows.) Apart from that you can just call them by their nickname and ask something related to what is we're (or they're) doing. For example, when I'm going to football practice and I meet them at the field, I don't go 你好，大家好 or whatever. It's usually something like: 嘿·~ 哥们儿，今天场地怎么样？ 嘿~ 三哥，你买了新鞋吗？ 马哥，怎么样？刚才下雨了，咱们还踢吗？

Actrually you can say more simply just like a word "hi" or "hey".And after that in different circumstance,you can use different sentence. For exemple: If you meet a friend who you had long time no see.You can say:"hi,好久不见，你最近在忙什么?" If you meet a friend who seems in a hurry.You can say:"hey,这么急干什么去？" There are many different ways to greet a friend in China,and sometimes depend on your relationship.^_^