Can't speak to much about the other CMSs, but I can say that it would be very hard to top Wordpress. The designers put a lot of effort into the out-of-the-box SEO. However, it is important to remember to change the permalink structure to a more SEO friendly one. Rich
Wordpress is a blogging software, you may use it with add-ons to work like a CMS but technically it is not a true CMS. Most CMS' are nowadays built good friendly SEO. The best one (from what I've seen so far) appears to be Drupal. However it is not the easiest to learn, although is one of the more powerful CMS' available.
Drupal, Joomla or ModX. Drupal does well, Joomla does well after some addons, and ModX does really well with mod_rewrite. It's usually not the CMS that causes good or bad SEO. I've seen sites still use dynamic URL's that have out ranked sites that have their URL's rewritten. Most of the SEO work is going to be on you.
Personally, I tried Mambo (Joomla) for a couple years and got tired of the extensions that tried to make the SEO urls. Plus, if a URL broke or a user got a 404error it would get logged and start filling up your database. Currently, I'm using Drupal and with a few modules installed I have everything I could ever want from a CMS. It's like being able to hand code each page with the SEO tweaks I like. With Drupal you can easily make SEO friendly URLS that you can change per content item. Or you can set custom URLs for different categories etc.. Drupal does take more time, patience and some coding knowledge to learn/use properly though compared to Joomla. You can also make the file extension anything you want. You could make your site appear more static by making .html extensions or if you like change it all to .php. -Jon
Joomla is fine for SEO up to a point. It allows you control over all the different metatags and there are good free SEO extensions available, including OpenSEF (though some say it is buggy) and ArtioSEF, allowing you to write friendly URLs. But be aware that URL rewriting can start to foul up once you add certain components which do not support it, and friendly URLs will not work. For example, I want to use Joomfish for multilanguage support, which DOES work with ArtioSEF (though not with OpenSEF), but I want Gallery2 on the same site, and the Gallery Bridge component DOESN'T work with ArtioSEF - in fact I have not yet found a way of bringing all these components together...
So they say. It will be a real hassle migrating from pre 1.5 versions though, as there is no direct way of upgrading. I expect someone will write migration tools, but still, it sounds risky to me. Also, isn't there some sort of trouble brewing in the Joomla camp? Some kind of major internal disagreements...