My favourite plugin in the WordPress Plugins Directory would have to be Yoast SEO by the people at Yoast. This is one plugin that I will include in all my projects. It is the perfect plugin to assist with writing excellent content on your site.
This plugin, since its inception in 2008, has helped millions of websites worldwide to rank higher in search engines than sites of non users. Yoast’s mission is SEO for Everyone. Their plugin’s users range from the bakery around the corner to some of the most popular sites on the planet today.
Yoast SEO Free contains everything that you need to manage your SEO, and the premium plugin and its extensions unlock even more tools and functionality for you to use.
For more information on this plugin you can go to its page in the WordPress Plugins Directory or my post about it and many others that I use in most if not all my projects.