It depends of your needs and perhaps the need of your community. But also it depends of people's opinion about softwares.
In general, you pick PhpBB3 for simplicity and SMF2 for features. Both are relatively popular and free, and since their current version came out they don't have security issues much anymore. PhpBB3 has fewer features and I think the theme editor is very limited, while SMF has more features. But SMF is not as common so it can take a while to get used to it.
VBulletin is not worth it IMHO, unless your forum starts averaging 1000 posts a day, and even then, many big communities do fine with a free software. Same for Invisionboard, which has a long history of severe security problems. MyBB and Alclmboard are free alternatives, but are not as known and I think MyBB is less secure.
any particular reason for that? what made you choose for SMF?
The reason why CodeWalrus chose SMF is because that's what Omnimaga used and since most CW founding members were from Omnimaga, we picked what they were most used to.
The reason why Omnimaga chose SMF in the first place (back when I ran it) is because that's what I was the most used with and prefered as administrator.
Now the most important questions, though, is what your forum will be about? Why are you starting a forum? Do you have plans to be active there in long terms even when activity is low and even with school/work/life? Because not only there needs to be demand for a forum, but there also needs to be an active administration with at least one admin averaging at 40% of the activity at first.
-EDIT: Nvm, I had forgotten that you were from Velocitygames when I wrote this, which already has a forum.