Social media, email, paid campaigns and partnerships all have bigger selling points (and a quicker RoI), but for a long-term solution, understanding the technical aspect of SEO makes the difference.
Whilst I wasn't fortunate enough to attend BrightonSEO recently, the organisers have been kind enough to share some great resources:
No one discipline of digital marketing is more more important than the other all of them need to be combined to provide the best possible results.