Drupal, Joomla and Wordpress are the top three CMS clients I suggest.
I've used both wordpress and drupal extensively.
I use Drupal for all my websites now because they have the most flexibility & options. If you are just starting out on website development, I suggest wordpress as a first-time CMS package.
EDIT: Just found this article today -
http://drupal.org/best-open-source-PHP-CMS-award-2009
PM me if you have any questions