Providing high quality relevant content pertaining to the end users query is the most important factor in SEO hands down. OK, back links are extremely important and I have spent years of my life getting them, but search engines are not in the business of serving up highly backlinked sites that have content that is not the most relevant to the end users query. Give them something valuable and you'll get the CTR that will defy all other SEO metrics. If you have 100% CTR for any given query you'll skyrocket to the top of the SERP regardless of backlinking.

As per my opinion On-page Optimization & Off-page optimization is major part in SEO. If your on-page is perfect then it will easy for you to get website ranking.

Major part of SEO is onpage and off page SEO process.