It depends on what you need your CMS to do.
From my own experience, Wordpress is by far the most flexible in terms of a blogging platform. It's such a flexible blogging platform, that it can be made in to an actual CMS for a general-purpose website and then some.
On the other hand, Joomla is an incredibly flexible "content" platform that excels more in creating general-purpose portfolio style sites that one would manage as a series of "articles."
From my personal experience, since I create a lot of general-purpose sites for clients that go beyond the idea of a "blog," I tend to gravitate towards Joomla more often (between the two) if I want something that is more flexible, because Joomla just has such a huge repository of plugins, modules and add-ons that can do so many things and perform so many functions, right out of the box.
On the other hand, when I deal with a client that is more focused on the creation of content, and their main purpose is managing posts and articles divided in to various subject matters, and organized in to various categories, tags, and taxonomies, Wordpress is really the only way to go, and there's nothing out there that can touch it for that kind of site.