Which are the most important SEO strategies to focus on?
Will Blunt from Bluewire Media recently posted an excellent article on their blog about SEO Strategy using their free SEO Planning Template which is highly recommended.
The three most important parts of SEO strategy are:
 good keyword research,
 on-page SEO, and
 off-page SEO.
All are equally important, but for different reasons. Read Will's article, and download the free SEO Planning Template, that will explain things much better than I can here.
Good luck with your SEO endeavours, but remember that the best SEO strategy nowadays is good, relevant, keyword optimized content. Google loves it, and so will your target market.