vBulletin all the way!! I started off with phpBB, but was sick of all the spam I was getting. Every morning, I had 10+ spam accounts registered that I would have to take care of. Granted, there are many more plugins for phpBB, but vBulletin has a good amount of features as-is.
I'm going to have to agree with deathshadow and the others about SMF - it's one of only two forum packages I would ever touch for PHP (the other being vBulletin, but I like spending my money on myself and my business, not on licensing fees). I would go into skinning and all that but deathshadow summed it up quite nicely.
phpbb if free, i do have hard time finding a decent skin for it tho. not sure about invision prize, but i like it a lot more, and imo it's best looking forums system ever.
any critics to mybb forum script? I tried to search for a free one.. I see vBulletin used more for forums but need to pay for license.