The best open source CMS?

I always prefer Wordpress, is nice, friendly and have a lot, a lot of extensions…And for the clients is very easy to understand. The only thing that Wordpress lacks is a good forum plugin. Wordpress.. yes, it seems too much popular but isn’t it just for blogs?
For clients that desire greater customization and more powerful functionality I recommend Joomla, but i guess it might be little bit tough for a beginner. Joomla! is extremely customizable, and with a lot of work it can even be SEO and WAI compliant. An excellent resource is http://www.opensourcecms.com/