Although the 'Keyword' Meta tag would seem to be the best and most logical answer to this question, we have come to learn that this is not likely to be the case.
Sure, nobody can 'know' which ranking factors are most important, because Google simply never publishes this knowledge. By trying different methods however, we created our own list based on best practices.
Factors that should be taken into account according to us are:
1) HTML Validation: The HTML Validation process checks for errors on a website, among which are errors that cause the site not to be indexed properly by search engines. So every site should be validated to ensure it is not losing rankings in the major search engines because of errors in the HTML syntax.
2) Correct url: The most important keyword should be in the url. Also the url should on the whole be built in a search engine friendly way, thus not using yourdomain.com/default.aspx?id=14 but for example yourdomain.com/news
3) META-Tag Title: Few people realize this, but we believe this is actually the major ranking 'content' factor, factors 1 and 2 are a bit more technical. The title should be built in the correct manner though, don't use too much characters. A key point in this is not using the domain name in an article, this is of no use and only lengthens your title.
4) Content: The content on your webpage is very important. Use the desired keywords in your content and make sure to have a good keyword density. Furthermore, it should be clear, easy to read and it should certainly be unique. Don't copy content from other sites, search engines can rank the 'uniqueness' and will take this into account!
When you have optimized your website and have the factors above covered, make sure you generate links to your website. Contextual backlinks (links to your website with the important keywords mentioned). The more sites link to you, the more important the search engines rank you.