Essential plugins for WordPress?

https://wordpress.org/plugins/wordpress-seo/ (Best SEO Plugin)
https://wordpress.org/plugins/google-sitemap-generator/ (sitemap)
https://wordpress.org/plugins/w3-total-cache/ (caching)
https://wordpress.org/plugins/wp-smushit/ (optimizing image files)
https://wordpress.org/plugins/wp-optimize/ (Simple but effective plugin allows you to extensively clean up your WordPress database and optimize it without doing manual queries: )

Security:
https://wordpress.org/plugins/wordfence/
https://wordpress.org/plugins/login-lockdown/