Here's my idea of Forums....
phpBB2, even if you have NO money, still don't go near it; it's horrible! Far too basic, and it has never given the name 'Open-Source' a bad reputation.
SMF, this is very good for first-timers who can't get anything else. It includes a basic plugin system, but Skin Templates aren't easy to tweak.
IPB, always a great system, so try and get a copy of the (Unlimited) Trial version if you can't get the real thing. Modifications can be a real pain to install.
vBulletin, ah, yes. Potentially the best forum system out there; all the major names use it, like Sony and others. Criticised a little because people think it's expensive, but If you had a choice between IPB and vB, vB is better Value For Money, and is cheaper in the long run.
Support if very good, and you have premium access to all the great Mods out there.
Plus, for the newbies, there is a Plugin System new for vB 3.5+ which will eliminate editing the source files!