mmm it's hard to make the choice but let's say that it depends to the target: 1- i need trafic for my web site: i will choose social media 2- i need high page rank : i will choose backlinks 3- i need to be present in SERPs: i will choose SEO but anyway i think if i have to choose only one tactic it will be: Backlinks
That's like saying your best tactic for eating food is putting it in your mouth and chewing it. This thread is totally pointless.