The best I've used is phpBB (phpbb.com); I had used phorum (from phorum.org) but that was a few versions back. w-Agora is highly customizable (
http://www.w-agora.net/en/index.php)
Check out:
http://www.hotscripts.com/PHP/Script...rds/index.html for a repository of PHP based boards (or change it to whatever language you prefer).
HTH