After installing and removing zillions of them, here are my faves:
Lazy loads images and other embedded media so that they are not sent to the client until the client scrolls near it. Saves bandwidth and makes for speed.
Automatic breadcrumb links on pages to ease navigation
Easily create and edit child themes based on themes already installed. This way you can update the third-party theme without losing custom changes to it.
Allows disabling of updates of specified resources (WP core, themes, plugins, etc.). Useful if you don’t want your client bothered by update notifications.
GA integration which inserts the tracking code on every page and also gives you a nice traffic history report on the admin dashboard.
Allows insertion of PHP code in the WP content editor using tags like [ insert_php ]
Creates a link to an unpublished version of a page or post that you can share with anyone. Viewer need not be a registered user or admin. Perfect for in-place-UAT.
Handles automatic metadata generation and tons of other SEO features.
Must-have enhancement for the WordPress WYSIWYG content editor which adds many features.
Automatic backup and restoration via S3, Dropbox, Google Drive, Rackspace, FTP, email, etc. I use this for weekly backups to my Google Drive.
Extensive security and virus-prevention package with automatic scanning and malicious user lock-out.
Helps WordPress cooperate with SSL/HTTPS-configured sites.
Makes Font Awesome and Genericons icon sets available in WordPress.