SteveWh - 6:47 pm on Sep 17, 2010 (gmt 0)
I still like SMF in spite of what could be its somewhat uncertain future. There was a severe dispute in the development team several months ago. I haven't followed it lately and don't know how it stands currently. SMF's security history was much better than the others for a long time. phpBB had many problems, but maybe they've got those ironed out more recently.
Secunia.com has a searchable database of historical and current vulnerability reports that you can use for comparison and making judgments.
Don't expect to set it and forget it, with any bb software. It is absolutely essential to install every update as soon as it is released. That suggests that ease of version upgrades is an important selling point. SMF is very easy if you don't make a million mods to it. I have no experience with the others.
myBB is one that seemed to get numerous positive comments from people who were looking for something to replace SMF.