I use both phpBB and also vBulletin.
phpBB is great for no cost and best of all, it really does run out of the box (as they say). you can have your forum up within minutes.
support for this free package through the official and unoffical sites is also quite good.
downside is that is can be fiddly to use and you will need to know a bit about HMTL for coding pages etc.
vBulletin is great. It also costs money but you do get some brilliant support (which after all is one of the reasons why you pay and how that money helps get you support)
also I like the way you can change things through the control panel and if you make a mistake, you can always revert back to the original version.
Leslie