From my opinion i will select IPB 2.0 or over. It is the best i've seen not as hard as vbulletin easy in every steps but it is from paid category and from free category the best is SMF Simple Machines Forum bcoz the main things that it has got is it had automated mods installations.. with in few clicks you can easily install and setup the mods completely.. you don't even have to touch the files to edit ony by one as phpbb. And this is the only features that other forum has not got yet.