How long should a piece of string be?
Some people may argue that the answer is that it depends on what you need it for.
phpBB would be a decent option if you had Linux-based hosting. I'm not sure how well it would run on IIS. I've always had weird problems with PHP on IIS.
vBulletin is the preferred paid choice of people wanting a paid forum but not being that technically inclined and want the "in thing". Ever since version 4.0 it has been complete bloated garbage and based on old web tech.
Have you seen that trainwreck that their 5.x preview is?