How do I structure an answer to a question about which factor is the most important?

Firstly, you need to decide how you are going to separate your factors. For some questions it is obvious how to separate your paragraphs but for others it is trickier. A classic way that history answers are separated are economic, political, and social. Remember, each paragraph has to be distinct; you don't want to put too similar factors in the same paragraph. You may have more evidence for some factors compared to others. That is fine, some paragraphs will be longer than others.
Crucially, you should not sit on the fence. It is important to show all sides of the argument but you must also evaluate. People differ as to where they put the factor the believe to be most important. Whilst this is not the most important, I personally prefer to put it last as it leads nicely into the conclusion.