BTW, I see that vBulletin, phpBB and IPB are the most preferred Forum CMSs. It is not a surprise for anyone, I suppose.
The best combination here is - functionality and look. Nothing more you can want from a forum CMS. The SEO - well, every forum script has its own modules, where you can achieve great positions in SEs, having in mind that SEs, tolerate communities, because of the useful and non stop updating content.
WP is now trying to take over this niche, but will be a hard task, that they will achieve soon, according to what I've been seeing last years from them.
Also, biggest forums use exactly of the 3 leading platforms from the poll.