The best free SEO module for Magento 2 is the Magefan SEO Toolkit for Magento 2.
It is a comprehensive SEO package that includes features such as automatic 301 redirects, canonical URLs, meta tags and more.
It also includes customizable options such as XML sitemaps and robots.txt editing, as well as support for structured data. This toolkit is an all-in-one solution that can help you maximize your search engine visibility and rankings.