Depends on your criterias:
Free:
PunBB is the prefered option, light, easy to modifiy and integrate, SMF is a good option aswell but I never tried it or potentially phpBB3 but still in RC (stay away from phpBB2)
With license:
IP.Board as here, very user friendly and the other products from Invision are very good aswell or Vbulletin but it has not evolved for nearly 2 years now.
One thing to take in account aswell is the server performances depending on the number of visitors / posts you have, IPB and VB can become resource hungry.