To me the search for the best content management system for the macintosh is a bit of a misnomer. If you want a good CMS it will likely work via the web (or whats known as cloud computing). Most cloud computing and software as a service applications I know work on both the PC and Macintosh platforms.
Using Open Source Content Management Systems
To answer the question, however, I would suggest that WordPress or Drupal is the best content management system if you are on a Macintosh. Both are free systems and only require minimal investment to get started. For instance, you should be able to create a content managagement system that works for you for under $500. For instance, with WordPress you can pick out a low cost theme like Revolution or Thesis and pay for someone who knows php to upload it to the server.
Using Wikis and Ning for Content Management
Another content management system that can work for users of Apple Mac products is a wiki (I recommend PB Works or Wetpaint–one caveat Wetpaint is not compatible with Safari, only Firefox). If you are publishing your content, but want on open publishing platform (in other words, one which is searchable by search engines like Google). You can also get private wikis, if you would prefer, however I haven’t researched those options. (I believe you can use Ning as a closed system publishing platform.
Content Storage and Collaboration with Google and Basecamp
Finally Google Docs and Google wav are decent as content management systems. However, it can be difficult (for me at least) to see all my documents in Google docs. Also, at a very low price point is Basecamp, which is one of the industry standards for project management and collaboration. Its great for collaboration on research and editing. In fact, Basecamp has a suite of online software products which provide idea management services.
You may also like this post where I compare WordPress, Drupal, and Joomla as content management systems for churches and nonprofit organizations.