Joomla, Drupal, and Wordpress have been ranked number two, three, and four respectively in the 2009 Content Management System Software Review by TopTenReviews.com. I don't know why they ranked "eZ Publish" since I have never heard of it, nor seen any books on it, nor heard anybody using it.
Obviously the answer to that question which CMS is best will depends on the nature of your website as well pass what qualities you place higher priority. As a general choice, personally I like Joomla. I find that Drupal has a much steeper learning curve. Wordpress is the easiest of the three to learn. Wordpress's user interface is slick with much AJAX action, is intuitive, and is user-friendly. Wordpress has the easiest process and the best user experience in terms of installation of the CMS, plugin installations, and software updates. However Wordpress is designed mainly for blogs. Although it can be used as a general content management system. One of Wordpress's strong point is its tagging capabilities.
Joomla and Drupal is designed more for general websites, but can have blogging capabilities.
Drupal, in particular, is good for creating social media and community websites as noted in the book "Drupal 6 Social Netwoking"
Drupal also is strong in taxonomy capabilities.
The Packt Publishing Open Source CMS Award for 2009 has not completed yet. But in pasts years, Drupal and Joomla vie for first and second place with Drupal coming in first in 2008. And Joomla coming in first in 2006.[ref]
2007 was a mixed year, with Drupal being "Overall Winner" and Joomla winning "Best PHP Open Source Content Management System".[ref].
