Mh, I highly doubt that Wordpress is the 'best' CMS, it has quite a few aspects that are not so great.

Popularity does not necessarily equal quality. :-) That's especially true for many things popular in the IM niche, isn't it?

But wordpress seems to be the CMS with the most plugins and themes available.
For Internet marketers there's a ton of plugins and specialized themes that you won't find for other CMSs.

About Joomla: I found it to be a mess when I tested it a few years back. Maybe it's better now. But I also find that IMers using Joomla build web sites with zero focus, zero usability and cluttered with dozens of distracting boxes and links that do not contribute to the site's marketing goal.

A commercial one worth looking is:
http://www.interspire.com/websitepublisher/

I also heard good things about Expressionengine, but never looked at it.
http://expressionengine.com/

Another popular and professional one is Typo3, but I disliked the concept that it has it's own programming language that you'd have to learn.

Ralf