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The most comprehensive I've used is Joost de Valk's Yoast SEO plugin.  It's absolutely the most comprehensive.  I don't use anything else now.  Prior, I was using Arne Brachenwald's Google XML Sitemap plugin and the All in One SEO plugin.  Joost de Valk had done a much better job of staying a head of versions and releasing a more comprehensive toolkit.

So is the solution for stem queries to attack offsite with increased relevant, promoted content? Or is it also 'fattening' relevant content on page?

This is the general advice that I continue to see that doesn't have any definition whatsoever... 1. What is keyword stuffing as defined by this algorithm change and compared to actual search results that have been promoted or demoted? 2. How can Google possibly define quality without investigating and rooting out specific domains... Which they insist they are not doing? 3. Define useful and not useful in a quantitative manner that Google can measure? 4. Navigation issues impacted sites before the change? Are you saying navigation issues (404s?) have MORE of an impact since?

I'm curious what correlations or impacting variables SEO professionals have found that have increased or decreased ranking with the most recent algorithm change. It appears that many innocent sites have fallen victim, especially larger sites. It also appears that Google is maintaining that specific sites were not targeted... Meaning there must be proven characteristics.

The most comprehensive I've used is Joost de Valk's Yoast SEO plugin.  It's absolutely the most comprehensive.  I don't use anything else now.  Prior, I was using Arne Brachenwald's Google XML Sitemap plugin and the All in One SEO plugin.  Joost de Valk had done a much better job of staying a head of versions and releasing a more comprehensive toolkit.