I would probably have to say Wordpress or Joomla. Joomla has a lot more capabilities because it is a multipurpose CMS while Wordpress is more of a blog CMS.
I just started looking at CMS packages and Joomla was the one I have settled on. It' pretty easy to pick up and very user friendly. The only downside to it so far is that I have found the official forum pretty unresponsive.
