Whichever your target demographic has grown used to.
For example, a webmaster resource forum these days really should be done using VBulletin because that is the 'standard' so far as what we are all accustomed to, many use phpBB and Invision however, those types of board script tend to have a slight negative effect on the amount of 'posts' a forum gets because webmasters are more comfortable posting on a board where they have used the forum script before, they know how it works and dont have to learn hundreds of commands again.
That being said, VBulletin is also one of the better forum scripts around at the moment if you are looking for a 'threaded' board.
You may also find this article of some use to you:
http://www.gaywidewebmasters.com/art...-scripts.shtml
Regards,
Lee