Kunena is the leading Joomla forum component. Downloaded more than 8M times in 11 years, Kunena is the only forum/discussion solution for Joomla that is community driven - true open source - public self-help forums, GitHub and documentation Wiki.
The most recent stable release, recommended for production websites running J! 3.9 , is Kunena 5.1.15
Details about Kunena 5.1 are contained in the release notes at https://www.kunena.org/blog