Best SEO plugins?

We're using Torvista's Structured Data Plugin, we generate an XML Sitemap and pump product data to the Google Merchant Center on a nightly basis as part of our Google Ads campaign - what's the best SEO plugin, especially for Google?

Going to jump in on the non software based aspect, quality information that a true customer would find beneficial, informative and unique is first and foremost. Gimmicks are more likely to cause down stream problems.

Besides that, what characteristics are you considering targeting to achieve "the best"?

Other things that routinely is stated is ensuring that pages pass html validation. There's also now Javascript/jQuery validation. validation. Not to mention accessibility such as color contrast/visibility based on the extra needs of some unbiblical. Image compression. Mobile accessibility, etc...

Again all that aside... (Readable/scannable) CONTENT.

This "SEO" topic has been discussed for many years and it's amazing how many "experts" there are in this field.
Interestingly, not one of these "experts" really has any valid advice - for several reasons:

1. Google (and Bing) change their algorithms frequently - sometimes several times a DAY. Over the years, so many changes have been made to not only the algorithms, but the underlying technologies that enable the internet and everything about it. What may have constituted "good SEO" yesterday, might not have any validity tomorrow.
2. The major search engines DO NOT talk openly about how their algorithms work - this is "secret sauce" stuff - so all these "SEO Experts" are just floundering around in the dark. They utter impressive computer-speak, and as a result, they hoodwink the ignorant in parting with money.

Perhaps the most important reference to SEO is what Google publicly states on how to construct and maintain a proper website, and if you adhere to the required principles and standards, then your website stands a better chance of out-performing competitors, and therefore have greater potential of higher ranking on searches.

DO NOT build/maintain your website for Google... Build it for PEOPLE. Google "emulates" real people and determines a website's value based on how useful and efficient it is to actual humans.