The
SEO battles are best fought using a "multiple front" approach.
Are keywords important? Yes.
Is the proper use of header tags important? Somewhat.
Is keyword density important? Yes.
Is link popularity important? Extremely.
Among other factors...
Will a massive effort applied to any one or two of these factors get a page at the top of the SERPS for a highly competitive search term? No.
With a non-competitve search term, proper use of any one of these factors can get a page a high ranking.
With a more competitive term, it takes better
SEO in one or more areas.
To rank well for the extremely competitive terms, every possible
SEO technique that you can think of (and maybe a few that you can't) has to be used in the best possible manner. Anything less in any one area will leave your page buried in the SERPS.
For example, while using header tags isn't the most important
SEO tactic in terms of effective
SEO, using them and using them correctly just might mean the difference between coming up #7 out of 1.5 million results and coming up #207.
SEO is an incremental thing. All the
SEO tactics work together to get pages ranked highly.
All of that being said, if someone were to ask what's the single most effective
SEO tactic that one can use (at least for the time being), it would be getting lots of inbound links with your target keywords/phrases in he anchor text. Nothing else comes close.