Wordpress was designed as a blog, not a CMS.
Joomla = The beginner CMS. Lots of plugins, lots of functionality, lots of slowness and problems.
Drupal = The advanced CMS. Not as much plugins, not as much functionality, used on very large sites and "easy" to modify.
Expression Engine = The paid CMS. Even less plugins, but it has a better admin interface and usability than the previous. Skinning is also quite easy.
Though to straight up answer your question, there is no best. It's what fits your needs and requirements the most. Wordpress can be scaled to handles thousands of concurrent visitors. Joomla can. Drupal can. Expression Engine can. They can all handle thousands/tens of thousands of posts. The only real difference I would find between them all is that you're more likely to find shitty exploitable plugins in Joomla and Wordpress more often than you would in Drupal and Expression Engine.