Hi John,
well that is exactly the topic most of the SEO's are fighting about: But in all honesty nobody really found the holy grail yet. If it's backlinks, content, likes on Facebook or whatever every so called expert swears in his methods.
So, I just come round and share my experience with you and what works best for me and my clients. That doesn't mean that it'll work for you at all;
On-Page
I think content is king No keyword stuffing, just a text which is full of information for the visitor and easy to read AND understand. Google (as you can measure on Analytics) recognises and records the time a visitor spends on your page. This is an important factor in my eyes as a visitor who is not interested in your information/services etc. won't spend 5+ Minutes on your page, would he?
Furthermore, good quality content shares easy. Interested visitors are sharing good stuff, logical. And visitors turn into buying customers if you convince them from your expertise and quality.
Off-Page
It's no secret that in regard to backlinks the factor quality counts way way more than quantity. When I create backlinks I take care that they are organic and from a blog, directory, website etc. from the same business branch as the website I am optimizing.
A website with an anchor backlink in a blog post of a high PR and High DA blog counts thousandd times better than 10.000 bought backlinks for whatever $ 29.99
Bottom Line
So my opinion is that ranking builds on logical factors. Google measures the quality which comes in (backlinks), how it comes in and what is in it at the end (content). If it's all on a high level it returns into sales/leads or whatever aswell as higher rankings.
Hope I could give you an idea and it helps,
Mickey