Personally I use Simple machines forums. http://www.simplemachines.org/
It uses PHPand Mysql like PHPBB as well.
I have created several websites that use it and with visual verification set to medium, I get maybe 1 spam account registering a month.
Plus it's easier to maintain/mod SMF than PHPBB, SMF is also less well known than PHPBB, so is less prone to attacks etc.
Just my 2 cents worth, I have used PHPBB as well it's nice enough, so I'm not trying to kick off a PHPBB vs SMF duel