WordPress most popular plugin

Plugin allow you to extend core of wordpress. Most popular used plugin are-

Show Custom form on website

Contact Form 7 : https://wordpress.org/plugins/contact-form-7/

For SEO

WordPress SEO By Youst : https://wordpress.org/plugins/wordpress-seo/

For Performance

WP Super  Cache : https://wordpress.org/plugins/wp-super-cache/

For E-commerce

Woo Commerce : https://wordpress.org/plugins/woocommerce/

For Easy Customization of page content

Advanced Custom Fields (ACF):https://wordpress.org/plugins/advanced-custom-fields/

Security 

Captcha : https://wordpress.org/plugins/captcha/

I will keep posting on important wordpress plugins. keep looking.