W3 Total Cache
W3 Total Cache is a performance optimization framework designed to improve user experience and page speed. It improves the user experience of your site by increasing server performance, reducing the download times and providing transparent content delivery network (CDN) integration.
Link: https://wordpress.org/plugins/w3-total-cache/
Disqus
Sick of spam, but still want to maintain a positive commenting experience for your readers? Disqus is an external comment system which makes commenting easier and more interactive, while connecting websites and commenters across a thriving discussion community.
Link: https://wordpress.org/plugins/disqus-comment-system/
BJ Lazy Load
Here is an useful lazy loading plugin to make your site load faster and save bandwidth. It replaces all your post images, post thumbnails, gravatar images and content iframes with a placeholder and loads the content as it gets close to enter the browser window when the visitor scrolls the page.
Link: https://wordpress.org/plugins/bj-lazy-load/
WP Smush.it
Definitely one of my favorite plugins ever, that I’ve been using since years. Once activated, WP Smush.it works in the background and automatically strips meta data from JPEGs so you’ll save a lot of space and bandwidth. Give it a try, you won’t be disappointed.
Link: https://wordpress.org/plugins/wp-smushit/
Backup
The plugin every WordPress blogger should have installed. Allows you to backup your blog files and data so you’ll never lose your site even in case of a big problem.
Link: https://wordpress.org/plugins/backup/
Better WordPress Minify
Better WordPress Minify allows you to combine and minify your .css and .js files to improve page load time. This plugin uses the PHP library Minify and relies on WordPress’s enqueueing system rather than the output buffer, which respects the order of .css and .js files as well as their dependencies. BWP Minify is very customizable and easy to use.
Link: https://wordpress.org/plugins/bwp-minify/
Contact Form 7
Any serious blog or website needs to provide a way for visitors to contact the site owner. Creating contact forms can be tricky, so hopefully this plugin is available. It allows you to create custom contact forms visually and integrate them in any post or page with no effort.
Link: https://wordpress.org/plugins/contact-form-7/
Watermark
Are you posting photography, or original images on your website? If yes, you might want to add a watermark to protect your intellectual property. Of course you can do it with an image editor like Photoshop, but this great plugin can do all the hard work for you. Give it a try!
Link: http://wordpress.org/plugins/alti-watermark/
Source: Cats Who Code