A good article on what are the primary ranking factors for websites is here http://www.davechaffey.com/blog/seo/update-to-seomoz-seo-ranking-factors/ The sumary was that the following percentages were what was though the search engines placed most weight on:

24% Trust / Authority of domain name
22% Link popularity of specific page
20% Anchor text of external links
15% On page keyword use
7% Traffic and click through data
6% Social
5% Hosting and registration data

Also out of the on page factors title tags carry the most importance, followed by keyword frequency then headings like h1 and h2. The bottom of the list of on page factors was the meta name and keyword tags which are not used that much anymore. Plenty more information in the article.