my preference for a full blown forum like this one is phpBB... for the same reason that JPnyc likes vBulletin.
Additionally phpBB is easier to hack into your personal needs if you have to do something "different" with it. I'm working up a phpBB site with blog areas and other goodies. The existing mods cover almost anything you might want, but tweaking is pretty easy. And... phpBB is open source.