I can highly recommend vbulletin, the setup is very easy, you just need to upload the files and know your database settings.
The support forums are very good with a lot of info and some very good mods constantly being developed.
A good mod for vbulletin is vbseo (http://www.vbseo.com) it optimises your forum for the search engines and creates sitemaps etc.
For a free forum I recommend phpbbseo – http://www.phpbb-seo.com It’s basiclly a modified version of phpBB3 but it comes pre-modded with a whole load of seo features.