The first priority is Purpose because it's the driving force behind the others. Focus and Development are next because Focus has to do with choosing and staying on a subject and Development has to do with saying something meaningful about it. Organization and Coherence come next, because they help make that meaning more accessible to the reader. A sense of Audience, in the particular sense, helps fine-tune this some more. And finally, Style and Mechanics are the forms that put a polish on the content. While each of these qualities are defined independently here, it is important to remember that they are actually so interrelated as to be inseparable. A weakness in any one makes it hard to get all the others correct.