Arjun,
There are lot of cool plugins to extend WordPress power. It usually depends on the need of end user. A lot of plugins may affect the Speed efficiency. Anyway, we usually recommend the following popular plugins,
Akismet
All in One SEO Pack
Contact Form 7
Digg Digg
Facebook
Facebook Page Promoter Lightbox
Google Analytics for WordPress
Google XML Sitemaps
Jetpack by WordPress.com
Lazy Load
Really Simple CAPTCHA
Responsive Select Menu
ShareThis: Share Buttons and Sharing Analytics
W3 Total Cache
WP Smush.it
DesignOrbital community is also welcome to share their cool plugins choice here.
Enjoy!