On the world of web, you can find a lots of open source content management system. For example, Drupal, Joomla, wordpress etc. I have worked with almost all of them. After working 2 yrs with Drupal, i have found that Drupal is the best content management system among all of them. Following are the reasons:

• Drupal is open source. That means, you can freely install Drupal and if you want, you can modify the source code to fit your requirements.
• Easy to use and install. No programming knowledge needed to setup a site.
• Drupal is modular.
• There are 2000 plus modules (pluggins) available for download.
• Drupal is flexible. You can easily turn your Drupal site into a forum, blog, wiki and many other types of web sites.
• Drupal has a very strong template engine. Theming of Drupal site is easy compared to other CMS like joomla etc.
• Drupal has a huge and very active community.
• Drupal has a very sophisticated, flexible Taxonomy System. So Drupal contents can be easily categorized.
• Clean-url feature of Drupal makes it search engine friendly. Also you can find a lots of search engine friendly contributed modules available for download.
• Drupal has a great access control system. This allows site administrator or owner the full control over user access permission by creating roles and assigning what they can and can't do.
• With Drupal, you can easily have multi-language feature in your site. That means contents of your site can be translated to one or more language.
• The big companies and organizations are now using Drupal.