According to the recent stats most number of people prefers to install Plugins for improving their sites SEO.ALL in one SEO pack is the most popular of all with downloads around 13 million.Some other most downloaded SEO plugins are Google XML sitemap generator,Google Analytics for WordPress,WordPress SEO by Yoast ,Google Analyticator and Broken Link Checker.
Akismet is the second most downloaded WordPress Plugin which clearly shows the intention of bloggers to stay away from of spam comments.
The Presence of plugins like Add This,Share Buttons by Lockerz and Sociable shows the importance given by bloggers to boost up the social engagements.
As expected Jetpack Plugin by WordPress is also on the top most downloaded plugins,which was downloaded more than 3 million times.
Here is a Infographic by WPTemplate.com which lists the top 30 most downloaded WordPress Plugins with User ratings and number of Downloads.