Top 5 elements that have the most significant impact on SEO

1. Crawler Accessibility: Search engines must be able to visit the web page. If the site returns an error code when the search engine crawler comes to visit then the search engine won't be able to index the site.
2. Quality content: There is nothing more valuable that proper, well written, relevant and unique content for getting ranking on search engine.
3. Title and Meta data: Though recently search engines have started giving less importance to meta tags (Due to black hat SEO), they still carry a lot of weight. The title of a page is the heading of what the content on the page is and should be relevant and concise.
4. Effective keyword research: Keyword should be selected based on the business. It is very necessary to understand the competition and do effective keyword research that describes your selling points.
After identification of keywords they should be spread throughout the page for e.g. Meta tags, Headers, Content, Alt Tags, Anchor links, etc.
5. Link Popularity: Inbound links should be from quality and reputed sites. It is more valuable when you business specific links. An oil and gas company would benefit more to get a link from Oil and Gas directory than some travel directory. Don't link just the home page, rather links should be deep within site too.
6. Clean HTML and use of semantic markup: Semantic markup is used to understand the document structure. It includes using headings, paragraphs, etc.