If there was a badge for the Most Important Factor in search engine optimization I would award it to the Title Tag. Even though good title tags alone won’t ensure high Google rankings, they can take your site a long way on their own. And without good title tags, all your other SEO work will come to very little.
Another reason they get my vote as the Most Important Factor is that they are relatively quick and easy to optimize. If you know your targeted keyword phrases and can edit your title tags (most content management systems will let you), then it just takes a few minutes to edit them. Once Google has spidered your site and picked up the change, you can check your rankings and/or your Landing Page report in Google Analytics to see what effect the change has had. Tweak, and tweak again, is what I say.