Mambo and Joomla are best in SEO
I have personally tried various open source software available for CMS and/or web publishing including wordpress, mambo, joomla, phpbb (and other forums) and I have to conclude to myself that best SEO services are offered by Mambo and Joomla
However, one must note that Mambo and Joomla by default doesn’t have great SEO features, and it is not even enabled by default. The basic SEO feature in Mambo and Joomla just organizes the URL friendlier for search engine to gulp, but doesn’t give any keyword benefit from content. When default SEO is turned on in Mambo or Joomla, the URL of content page looks more like www.seodon.com/section/cat/id.
This misses out the big on keyword advantage in URL (major SEO factor, see list of SEO tips). However there are solutions available to overcome this easily. OpenSEF, 404 Advanced and few more commercial/open source components makes URLs look like www.domain.com/section/categoary/content_title.html
Having discussed this, one must question about what is big deal having this in Mambo or Joomla as same is available (and with fewer efforts) in wordpress, blogger. Though blogger and wordpress (and others) are very easy to use, easy to publish and SEO friendly (URL), what I found missing was ability to edit meta keywords tags of each and individual page at the time of publishing. There is no way in wordpress or blogger to specify keywords, description meta tags for each page published, all one can do is define global meta tags (you need to edit templates for it).
The beauty of mambo or joomla lies in its feature to let user define their meta tags (and if you are not sure, just include page title as meta keywords) right at the time of page edit.
In all my experiences this has been biggest positive of joomla or mambo over wordpress, blogger ( I have migrated from blogger to wordpress recently).
My SEO Friendly mambo site: www.PhotoGrow.com