i am using smf, it is good and lots of peoples are using. It has lots of mods also. and too it is free.
As I know VB is the only forum software that has itrader feedback option. There are many features in VB that you cannot find from others.
I-trader is an addon, it does not comes bundled with vBulletin by default. vBulletin is by far the best.
Like many have pointed out, VB has the itrader feature. Apart from this, I think phpBB is pretty good due to the support for it.