Here’s a list of some of the most popular plugins and are often used with most WordPress installations.
Contact Form 7 is one of the most popular plugins for WordPress and is currently active on over 5 million websites. It is completely free but does not have a drag and drop builder like some of the premium form plugins.
Yoast SEO is the #1 WordPress SEO plugin and is active on over 5 million websites. The plugin has a lot of great features to ensure you use best practices when crafting SEO friendly pages, posts, and content. It is also a great way to learn about SEO.