I despise SMF. The software is really tacky and very insecure. MyBB and phpBB are by far the best free forum software options. I prefer MyBB because the plugin system is fantastic, as is the community and there are some great themes.
I like the new XenForo which is paid but is the best forum software I have seen yet. Vbulletin is also pretty good. But for free forum software that I like I have to say MyBB It is easy to customise and install new themes. However, they could improve there addons installing system to make it slightly more "automated"