CMS platforms provide enormous potential benefit to SEO since they allow site owners to prime the organic content pump. And a well-made dynamic platform will out-perform ‘flat’ html sites because structural SEO features (meta, page titles, urls, sitemaps, etc.) can be easily – and constantly – managed from the backend.
Here's how the 3 major platforms stack up in my exp[erience:
Wordpress: I think most developers would agree that Wordpress is the premier blogging platform in the world. It comes with a very deep inventory of extensions, is a snap to install and set up, and very easy to add/edit posts, manage comments, and add nifty widgets. Strengths: blogging. Weaknesses: navigation is best suited for categories and tags rather than pages. Plugins do not always work in harmony. Template structure relies on multiple files, making updates a bit harder.
Drupal: a rock solid CMS platform, Drupal was the first major platform to make structural SEO a standard feature. Strengths: Easy to use backend, lots of extensions, and an excellent portal system. Weaknesses: installation/configuration of core functionality and add-ons is not straightforward.
Joomla: the most popular CMS platform in the world, Joomla is used for an enormous range of applications and organizations ranging from Ebay to the UN. Combined with a well-coded template, JCE WYSIWYG, and the K2 CCK, it offers all of the advantages of Wordpress and Drupal. Strengths: 6,000+ extensions, flexible modular approach to layout. Weaknesses: The default themes are weak and poorly optimized. Like the other platforms, it is best handled by a developer/team that has expert knowledge of the pitfalls.
In addition to Joomla, Wordpress, and Drupal - the 'Big 3' of CMS (Content Management System) platforms - there are a plethora of other Open Source and commercial CMS models and products. On the Open Source side, ModX CMS deserves mention for its outstanding quality and zeal for search engine-friendly architecture. Development costs in ModX tend to be higher, and in our experience the backend is not as user-friendly. On the commercial side, the best known platform is Expression Engine, which provides a fair amount of functionality (not as much as Drupal or Joomla, and certainly not for vertical markets).