Which SEO technique is most important?

There are so many SEO techniques. If you were to pick one SEO technique that generates the most results, which would it be?Is link building the most important? How much more important is link building compared to the second most important SEO technique?

Somebody says building links is probably the most ...

Link building is clearly the most important,
I'd say links are the most effective currently, in terms of SERPS.

social bookmarking and forum posting are also the effective one.