I would conclude that if an article I publish rises to #1 on page 1 for the article's most obvious keyword then that article has the best SEO possible. Firstly, would you agree with that statement? I have a number of articles that attain this. However, I have others that attain it and then plunge right down the rankings - sometimes so far down that I can't find them. What explains that? BTW I only work with Google SE
Terrible assumption. Why would you "assume" that a new article that likely has practically NO backlinks (except those from the site where it is posted) has the BEST SEO compared to other URLs that have legitimately ranked on page 1 for a long time and already have lots of backlinks? Google frequently gives new pages that it discovers an artificial boost in rankings when they first discover the page. This is sometimes called the Google honeymoon. No one knows for sure why Google does this. Perhaps they do it as a way of getting fresh content like late breaking news to the top of the SERPs. Perhaps they do it to gather some metrics about the page that can only be obtained if users actually SEE the page in the SERPs. Regardless of "why" they do it, after a few days (some times even a few weeks) Google stops artificially boosting the rankings of that URL and the URL falls WAY back into the SERPs where it "should" have legitimately been ranking all along.
some of my articles maintain #1 page 1 and in so doing beat articles published and presumably optimised by prominent national newspapers? Google should like what I write as it is 100% original but why does it go off some of it but sustain the rest?
You have to stop guessing and be proactive. Keep it at the top or get it back by refreshing/adding content and constantly be building backlinks with every legitimate method available.
my take is, if u optimized on only one type of SEO, u will see rapid decrease in SE ranking.. for me, i do many types of link building, many forms of optimization and my methods of white hat methods to make sure i generate traffic from different sources.. so that when search engines changes algorithms about some techniques, u will still have heaps of visitors because you optimized in many areas. that's my 2 cents, but it's not an easy task though.. but in the long run, unique content, regular updates and patient SEO actually pays off..