People I know always ask me that question and I always give them the same answer...
The most important part of SEO is the one you're working on!
Meaning, linkbuilding is the most important one if you've already selected a great keyword! On-page optimization is the most important, if you haven't done any...
My answer is that any ONE of them can be MOST important, depending on where you are in the process. Skip over a couple of simple things and your hard work goes for naught!
What is the most important factor in SEO ,Such as link building,Meta tags so on.
http://www.seomoz.org/article/search-ranking-factors
we can't live without water but it's not the most important thing in our lives.Most important factor is "Text"... it is nearly impossible to rank well without it.
You never "know everything" and let me tell you my friend :
you never loose a war, you let your opponents win
That about it and that's all
Backlinks are #1, and domain authority #2. I've ranked domains with no relation to the keyword, and virtually no content (and defintely not unique) well for many thousands of keywords just with backlinks.
hope this is fine for you...
Code:http://www.seomoz.org/article/search-ranking-factors
Domain age is not an important factor....anyone who says it is doesnt know what they are talking about. If they want to argue the fact I'm willing to display tones of URLs which show that a 0 age, can rank above a 10 year age for any given keyword.
For me, the most important are;
domain name
keyword density
tags
backlinks
NOT in that order.