What is the proper way of writing an exclamative question:
- What are you doing!?
- What are you doing?!
or is it better to just leave it as a simple question?
According to the Interrobang wikipedia page, this symbol [‽] has been created to convey the meaning of your question. Using it you ask "a question in an excited manner, express excitement or disbelief in the form of a question, or ask a rhetorical question."
But, always in that page, you can see it's a nonstandard symbol, so like it says, "in standard English, the same inflection is usually notated by ending a sentence with first a question mark and then an exclamation mark."
Now, it's also true that in an informal context, most people won't really care about it, but if you must choose a "line", it's that one.
The question mark and exclamation point should not be used together as in your examples 1 and 2.
The simple solution here would be to use the exclamation point only, treating the sentence as an indirection question:
Yet, there are other ways of indicating emphasis. Consider italicizing the key word (from your perspective):
Or, you could employ additional text for clarification, e.g.: