There are no hard and fast rules about what greetings (salutations) to use in email. If you want to be polite and formal in English, then you should say something like:
Dear Mr. Smith:
Dear Ms. Jones:
Dear Dr. Jekyll:
Dear Prof. Einstein:
If you want to be polite and informal, replace the colon (:) with a comma:
Dear Mr. Smith,
When authors send me a manuscript, regardless what salutation they use to address me, I always start off this way:
Hi, Dr Jiang,
Hi, Prof Jou,
(Almost all my clients are Taiwanese doctors and medical school professors).
When I write to my friends, I always start of the same way:
This is just a stylistic choice, a personal preference.
For business letters other than my editing, however, I always use the polite formal style in the first examples.
There is no definitive answer to your question, just a recommendation that for serious and important business letters (anything not to a friend or a penpal, and anything not informal), that you use the polite formal or polite informal style. You can find lots of choices in handbooks about how to write letters in English. Be advised, however, that different brands of English prefer different styles. The one that's best is the one you feel most comfortable with and the one that you think works best for any particular letter.
The more formal and more polite your initial salutation, the more emotional distance you put between yourself and the person you're writing to.
For friends and otherwise informal emails, the same rule of thumb applies: The one that's best is the one you feel most comfortable with and the one that you think works best for any particular letter.
Everyone has a different idea about what's good.