My favorite open-source CMS is WebsiteBaker. It's easy to set up and extremely easy for the site owners to maintain, and it uses short, SEO-friendly naming convention for the URLs. Designing custom templates is also very simple -- just a few PHP snippets included in standard html files.
Joomla is very user friendly CMS, and of course it's SEO friendly, it's module/plugin, you can find easily and it gets updated and very useful....
appearance of the site's on this CMS are beautiful...
I am also a Joomla user and big fan. I agree that this CMS is user friendly, but it can be overwhelming for new users. Just take your time to mess with it first before deciding to give up.