If I were you, I would just follow search engine's standards in SEO. For me this is the best way of promoting your site. To make it clear, never ever make anything that will cause SE's to punish your site.

For me it was on site SEO. I feel it helps me do less work in the back link department if you do it right!

If I had to only pick one I'd choose building relevant incoming links from relevant and trusted sites as this seems to carry more weight than anything else.

Of course I would add that concentrating on just one SEO tactic is not a good idea because you can bet your competitors will be taking a more comprehensive approach.

links from social media. Because this helps to get good ranks and referral traffic

social bookmarking is the most effective SEO technique for driving healthy human traffic and for productive crawlability.

i will go for diversified backlinks and write original quality content