There are way too many extensions in the Joomla directory to truly make a Top 10 list since so many of them are excellent.
We do, however, have some favourite we thought we'd share in order to help them get some more exposure.
Here they are in no particular order or ranking:
#1 - JomSocial
JomSocial is an excellent addon reminiscent of Community Builder in that it provides some very cool add on functionality that turns Joomla! into a more social environment. JomSocial has quite a few options and we find it a bit more attractive in that it seems to blend well into the system and provide for a more attractive look.
#2 - JoomlaFCK Editor
Easily our favourite when it comes to editors. This one simply blends nicely and has the type of look and functionality we love. Speed wise, it's an incredibly fast system for a freebie.
Website: JoomlaFCK Editor
#3 - ccBoard
Incredibly good integration with Joomla! make this an amazingly functional choice for your forum needs. It is fast, attractive and has a ton of options. Easily one of the best available forum platforms available today for Joomla! It is very reminiscent of phpBB3.
#4 - RokSlideshow
Beautiful plugin with very nice slideshow capabilities. At the moment it can be a tad resource intensive but still one of our favourites.
#5 - Exposé
Gorgeous flash gallery implementation by GTek. This is the one you want if you are looking for a fast and stunning flash gallery plugin.
#6 - Ninja Ajax Banner
We like the idea of this plugin and it's execution. Very cool to have the ability to have hover over banners and place 3 in one spot.
#7 - AllVideos Reloaded
Great plugin with very solid functionality that provides the ability to add videos in various formats to your site.
Website: AllVideos Reloaded
#8 - JFusion
Exceptional user authentication integration for Joomla!. Supports a number of products such as: vBulletin 3.8.0, phpBB3, MyBB 1.4, SMF 1.1.7, magento and Moodle