This is my favorite SEO extension. It allows me to manage SEO metadata from the frontend very easily. Also for sites that I build using Yootheme Pro, it makes access to the metadata section very easy.
In the backend, many advanced options are available, but a lot is done automatically, like providing correct canonical URLs, and generating Open Graph data. It constantly analyzes your site to detect changes, identifies important pages, adds meta and structured data, optimizes for social sharing, and builds your sitemap.
4SEO also provides you with tools for everyday SEO work: a list of broken links, custom error pages, redirections, canonical tags, content replacement, robots.txt, and more.
Finally, it connects to Google Search Console to inform you about keywords you should use and pages you should link to.