Universal greetings

4

Use them anywhere, anytime. The absolute standard.

你好 nǐ hǎo hello (standard) (literally "you good" — the default greeting)
您好 nín hǎo hello (formal) (您 is the polite "you", used with elders or in pro contexts)
你们好 nǐmen hǎo hello (to several people) (for greeting a group)
大家好 dàjiā hǎo hello everyone (when addressing an audience)

By time of day

5

Warmer and more specific than 你好, especially used in friendly or morning contexts.

早 zǎo good morning (short, friendly form)
早上好 zǎoshang hǎo good morning (full form, more formal)
中午好 zhōngwǔ hǎo good noon (rare in speech, mostly written)
下午好 xiàwǔ hǎo good afternoon (rather formal)
晚上好 wǎnshang hǎo good evening (from around 6 p.m.)

Informal greetings between friends

4
嗨 hāi hi (very informal) (borrowed from English "hi")
嘿 hēi hey! (mostly to get attention)
吃了吗 chī le ma have you eaten? (traditional greeting, like "how's it going?")
最近怎么样 zuìjìn zěnmeyàng how have you been lately? (elaborate friendly greeting)