I am not a big fan of vBulletin as it's administration interface is (for me) difficult to navigate through. I have used IPB (Invision Power Board) in the past and had no complaints. Quite frankly, if I had the money to purchase the latest version you best believe I would! I currently use SMF (Simple Machines) on my forum- it does the job so I have no complaints. Although it has a ton of missing features compared to vBulletin and IPB. phpBB on the other hand is quite horrible. I use it as a "PD forum" for a server simply because it's free and allows groups to be board moderators.