I used to be in love with phpbb... After switching to VB i have changed my mind. Its completely on the different level.
Ummm....not sure what the thread creator was thinking...the poll asks for forum software, yet he asked about portal in his first post Using vB for forums and Drupal for my "portal" and I am happy as can be
vBulletin easily, but not everyone wants to pay for it. You can't go past phpBB for a free solution, just ensure you keep it well updated as it can be buggy.
simplemachines.org Its the evolution of the YaBB SE project. In otherwords, its a php version of YaBB but slightly more advanced. Not as good as paid options obviously, but its one of the more noteworthy free options. Its generally regarded as the fourth major BBS for a reason.
I have been using IPB for a number of years now and so when I try a different board, I'm pretty biased since IPB seems more easy to me. Although I wish Invision would update their stuff more often.
I can't really choose just one... my top 3 are Vbulletin, IPB and PhpBB... I voted for IPB though because I'm currently using it right now.
Since I have 3 sites in pre-launch, technically none at the moment. Once launched: 2 vBulletin 1 CX-Board