According to you which is the best free (Open Source) forum software? Is it SMF, myBB, phpBB, vanilla or any other?
Points to consider:
Security
Functionality and ease of use (autoupdates, plugins, extensions)
Customization (Themes)
SEO and Integration with CMSs like Wordpress and Joomla
If you are voting for a software, please explain why do you think it's the best.