Kunena (Joomla plugin) is also free.
I've been going through this exercise for an existing forum that is thinking of going away from phpBB. Others considered have been Discourse (as now used by the MX5 Owners Club) as well as Xenforo (Vincent Owners Club). I'm not sure what Thumpertalk https://thumpertalk.com/forums/
uses but it's pretty impressive (it uses an AWS clone for attachments after server side image compression for example).
It depends a lot if the organisation wanting to use a forum have people sufficiently IT savvy to install, maintain and administer it.
phpBB is great if you do (kunena isn't bad either). The 'turnkey' solution offered by Discourse is superficially attractive if money is no object.
Still working on my report. The organisation relies a lot on a commercial website supplier and just volunteers for the forum (except mySQL level and server script FTP level access, which complicated things).