From what I've seen, vbulletin and phpbb are both nice. I am a member of a forum that uses vbulletin, and as a user, it's pretty nice and good looking. I'm also admin my own forum, and I used phpbb. Because it was free, and because I thought it was strong, stable and easy to use.
i like phpbb, cuz it's free ~wink~ i din't much care for some of the other free software. some looked like cheap phpbb knock-offs, which seems pointless. some didn't strike me as secure. SMF just didn't feel right to me, but i didn't play with it long enough to remember why i opted to not use it.
i have played a little bit with Vb and with IPB. i prefer Vb slightly -- the permissions seemed a lot more adjustable -- but they strike me as somewhat similar.