Here are the best WordPress plugins that will add new functionality to your WordPress site. These all are free and open source plugins and you can download them from the official website.
When you use any plugin for the first time or just updated to major version, make a backup of your database. It is always the best practice to make a database backup before using any plugin for first time.
1. W3 Total Cache : Best caching plug-in for WordPress and it will improve your site page speed. It lets you to minify scripts, cache database queries, add expiry headers to static images and more. It also reduced page load time, increase visitor time on site and visitors view more pages.
2. Yoast SEO : It helps you to create better titles, block duplicate pages from search bots, add meta tags required for more search friendly Google snippets. It also integrate Twitter Title Tags and Facebook open Graph tags.
3. Redirection : It manage 301 redirections and keep track of 404 errors without requiring knowledge of Apache .htaccess files. If the URL of an existing post has changed or if an external site is linking to a non existing page on your site, You can create redirection rules and automatically forward all the incoming traffic to the right pages. There is an good alternative for redirection, known as Simple 301.
4. WP Optimize : It helps you to clean up WordPress database tables and optimize it without doing manual queries, also remove post revisions, trash spam comments. It optimize your MySQL tables so that it works more efficiently and consumes less space.
5. Adminer : It helps you to manage your MySQL database from the WordPress dashboard itself. Adminer is a full-featured MySQL management tool written in PHP. You can perform database optimization, run SQL queries and browse tables.
6. Search Regex : This adds extremely powerful search and replace capabilities to your blog. You can find and replace almost any kind of data stored on your WordPress site including Meta data, post titles, comments and excerpts.
7. SlideShare : You can install this plugin to easily embed SlideShare presentations in blog posts with the help of short codes. This may be optional as WordPress core itself supports SlideShare embed.
8. Smart YouTube : It lets you embed YouTube videos in blog posts using standard video URLs and the embedded player can be customized. You can also embed Vimeo, Metacafe, Liveleak and Facebook videos and play in normal and HD mode. It adds a thumbnail of the embedded video in the RSS feed.
9. Yet Another Related Posts Plugin (YARPP): This plugin adds links to old stories that may be contextually related to the content of the current article. It also helps expose your archived content to search bots and display related posts in RSS feeds with custom display options.
10. Authenticator : Enable 2-step authentication for your WordPress blog, similar to your Gmail account.
11. WP DBManager : It Allows you to optimize database, repair database, backup database, restore database, delete backup database , drop/empty tables and run selected queries. Supports automatic scheduling of backing up, optimizing and repairing of database.
12. Debug Queries : Use the Debug Queries plugin to determine which MySQL queries are taking more time to execute and thus slowing down the blog.
13. P3 Profiler : This plugin creates a profile of your WordPress site’s plugins performance by measuring the impact on your site’s load time. Many times, WordPress sites load slowly because of poorly configured plugins or use of many of them. P3 Profiler generate a pie chart detailing the time it takes to run each plugin while loading the various pages of your site.
14. Google XML Sitemaps: This plugin will generate a special XML sitemap which will help search engines like Google, Bing, Yahoo and Ask.com to better index your blog.
15. Pubsubhubbub : When you publish a blog post, it will instantly ping Google and send a strong signal to the search engine that you are the original author. This plugin is widely adopted and is used by Google Reader, Google Alerts and many other services.
16. Limit Login Attempts : This plugin logs the IP address and the exact date and time when a user is trying to log into your WordPress and keeps tracks of unsuccessful login attempts. You can automatically lock out users after they have made ‘n’ unsuccessful login attempts.
17. Akismet : This plugin checks your comments against the Akismet Web service to see if they look like spam or not. Each comment has a status history, so you can easily see which comments were caught or cleared by Akismet and which were spammed or unspammed by a moderator.
18. Contact Form 7 : It can manage multiple contact forms and support customization. No need of coding with the Contact Form 7, as the forma and mail contents can be easily customized through the settings. The form supports Ajax-powered submitting, CAPTCHA and Akismet spam filtering.
Comment below if any other most essential WordPress plugins which is not listed here.