What is the most important factor in an SEO campaign?

If it had to be narrowed down to one then the key metric would have to be your Domain Rating (Domain Authority).  Albeit, domain authority relies on the accumulation of many different factors it is the most important factor to pay attention to because the basic summation of it is that it says how much your site can be trusted and therefore will determine how high you rank on Google, Bing, and Yahoo.