Well it depends on how deep your pockets are. Having used phpBB extensively, IPB fairly extensively, and vBulliten somewhat extensively, I can say that my fav is IPB. ITs just more lively and clever than vBulliten, and I think this has to do with the fact that IPB is a little smaller than vBulliten (or Jellsoft, sorry). And I think IPB is cheaper too. Plus, their support is great. I called them up one time and the chick on the phone was great!
However, if you want something quick, cheap and effective, phpBB isn't really all that bad. It just doesn't seem to get too many updates, or didn't way back when.