It's really hard to say that one particular task is the biggest factor in a web ranking
. There is on page and off page optimization. Nobody really knows what methods search engines use to index your website. But we can do research to see what works best for other sites. So if I was going to narrow the most important step down, I'd have to say it would be the amount of back links that you have going to your website.
Backlinks are the World Series in making it to the top in the search engines. Go out and get a lot of links going back to your website and you can just about rank well for any keyword you are trying to target.
As stated before, the search engines don't disclose what their ranking methods are, but I feel it is getting harder for website owners to obtain a good amount of links going back to their website. I don't think the search engines look at reciprocal linking and buying links the same way as they did in the past. I believe that the search engines are looking for more quality links that go back to your website and that is why this linking process can be a bit difficult.
I believe search engines are after the quality relevance over a long period of time. So you still have to concentrate on back link building whatever way you do it. The most important factor in your linking plan is that you stay consistent.
The thing that I do to get good backlinks to my website is to write and submit articles.
You want to do things naturally, so going out and posting hundreds of backlinks to a website that just when live is not a good thing to do.
When you go to your keyword list, find the one that appeals to you the best and make that your primary domain name. Not everyone will link to your website using a keyword. Some will just link back to your website with the URL. If this is the case then the search engines can figure out what the main subject of your site is. Don't overlook putting the keyword in the title, description and keyword meta-tags. Just those two steps alone can rank you number one in the search engines with the keyword you are trying to target. Just make sure you don't go out and get those links going back to your website in one day. Web rankings take time to build up.