Well I have used phpbb2 and phpbb3 and SMF (Simple Machines Forum). None of which do an excellent job at being
SEO friendly.
I think of all the forums, VB is probably the easiest to make
SEO friendly.
I am about to launch a website with inbuilt forum that should be very
SE friendly simply because it will be missing half the useless functions most forum software programs come with.
I think generally speaking forums only provide small value to search engine results and if you want your website to be found you need to add something like indepth articles and reviews etc.. to really grow in search results.
There are of course exceptions to the rule, but for the most part I'm not too worried about search engine friendly forums on my own websites.