I have played around with different forum software and am partial to vBulletin.
However, if you want free, I'd go with Mybb, Invisionfree, or Proboards, since they all seem relatively professional and offer a good degree of flexibility JMHO.
Btw Vbulletin all the way if you have a few bucks. Get a free 24 hour trial board on their website and play around.