vBulletin, IPB, phpBB is most common forums (not CMS) software in my practice. I would recommend using it, if you have some hundreds and more of active visitors, otherwise other two choices will serve you as well. With phpBB being free software and IPB cost at 149.99 USD (vB v4.0 is 195 USD) it also depends on your budget. Do you also have a website and forum will be an additional method of site promotion or it's intended to be a standalone product? - do you use blogs of any kind already?
I don't have a license for it but I have set it up a few times. It's easy to install and get up and running but customization is a hassle. Maybe it's because I'm not that experienced with it yet. Anyway, trying to integrate a completed design into the CMS took me a couple of days and there were still some things that I missed. I'm not a big fan of the forum part of the script either. I like vb3 much more. Maybe that's just because I have been using that forum script for so long.