The truth is there is no proven evidence that states one is more important than the other, however, most people tend to think offsite (link building) surpasses the other. If that were true then how do you explain the fact that you can build links to a site until your face turns red, but if don’t have your onsite SEO in place then link building can be fruitless.
Some of you still don’t seem to think so, right?
Let me put it this way – If all your on page factors are in order and you only have a single back link or didn’t do any offsite SEO to your website, then you have everything that might take you to number one on Google, provided that there are not many competing websites. Yes, you read that right. However if the competition is high for the words you want to rank for, then you will need to do some offsite SEO depending on the amount of competition.
So in the end, I guess getting good search engine rankings is a matter of doing both onsite and offsite SEO but I strongly recommend getting all your onsite factors right first before worrying about link building.
Below are few random basic onsite SEO factors that should be implemented on any given site:
Title tags and Meta description tags
Title tags and Meta description tags play a vital role in your on-site SEO activities, not only because they tell the user what the page is all about but also the Search Engine bots – so when writing them make sure they are eye catching, interesting and keyword rich. Although this will not have a major affect on your SEO rankings you will benefit from an improved click through rate.
Quality Content
As we all know content is king and it drives the web – Adding a news section or blog to your site is one easy way to keep your content fresh by adding at least 1 post a week. Remember search engines love unique content. Make sure all your page content is truly newsworthy and that’s it.
Internal Linking
Implementing a well structured internal linking strategy between relevant areas of your site will ensure that you show Search Engines that you are aware of how your pages should relate to each other.
Duplicate URLs
Do you have three or more versions of your home page http://www.site.com, http://site.com and http://www.site.com/index.html that Google could be indexing? If yes then redirect them to the one with the most authority otherwise you will be penalized by Google for duplication issues even if you have thousands of back links to your site. Yep that is also true.
Broken links
Are there broken links on your site? A check to ensure that there are none is vital as Search Engine bots hates broken links and it will slow down your progress. If you don’t have Google Webmaster tools verified, now is the time. It has a list of all 404 pages on your site.
Static & XML Sitemaps
Sitemaps help Google bot discover and index pages on your website faster.
Robots.txt
Use robots.txt to restrict pages that you don’t want Search Engine bots to crawl or index.
See that wasn’t too much to bear was it? Well don’t get too comfy as there are numerous other elements to take into account from an onsite perspective. When you have all these are in place well it will then be a good time to start with your offsite SEO.
Because in the end all we want is that top position in the Search Engines and Search Engines are just a super computer / algorithm which tries to make sense of all the text on your site (onsite factors), links and text from other sites (offsite), and if you connect them both, you will make the job easier for search engines to give your site that top online positions.
I never knew how important link building was,thank you for the interesting and insightfull post!
This is exactly what I am trying to explain my clients for years now
Off site isn’t something you need to worry about if you don’t work on your on site optimization properly. Indeed, we can’t rely only on the on site part if the competition is “huge”, but nobody declines our right to optimize for many long tail / low competition and receive many clicks. In my experience, I have noticed websites that have ranked for multiple keywords without any S.E.O., only because they were relevant, had tons of original content ( Panda is here 
) , and a FEW high quality back links that helped them with their Page Rank / search engine rankings. Link building is important, I won’t deny it, but working on site gives someone more control and lower expenses – no matter the niche, after all we have billions of keywords we can target 