phpbb is the best, for the reasons elio stated.
OpenSource beats anything else
LOL just downloaded punBB source and it has no templating system. I mean how bad is embedding HTML code in PHP.
@OP - Custom coded forum would be the most secure IMO but from the choices phpBB.
// @define('DEBUG', true);
// @define('DEBUG_EXTRA', true);
@define('DEBUG', true);
// @define('DEBUG_EXTRA', true);
@define('DEBUG', true);
@define('DEBUG_EXTRA', true);