Joomla is if u wantr something that u ll get support for almost everything u like, u ll have a tons of templates to choose, and many developers to help u.
It has a learning curve of 1-2 motnhs imo. Suitable for all kind of sites....

Drupal is easier to learn and to edit, it has better seo option(buid in) and it can have unlimited tree categories(joomla has 3 for now), also u can have many type of user rights and groups. Suitable for communities.

Wordpress is not a cms, is cool blogging script that its very popular so u can find many solutions that can turn into a CMS. Suitable for not experienced users.

ps: there s also a CMS more advanced calle typo3, i m putting it here just for your knowledge.