The top open source CMS in the market is Joomla, Drupal, and Plone. Joomla is easy to use, set up, and maintain. Joomla can be installed in basic shared hosting. Its installer looks much like the simple desktop software. The administrative interface looks much like a desktop program as well.
Drupal has the combination of power and ease of use. It is built in PHP and can be hosted in a basic shared hosting. Drupal needs more more of a learning curve than Joomla, but offers more functionality for sophisticated websites.
Plone can be used to create a powerful world-class applications. However, the learning curve for web developers is substantial. Plone also needs special hosting requirements. Plone can be a great choice for meeting sophisticated website needs.
Im summary, Joomla is the easiest to use, and plone is the most powerfull. Drupal stands between them.