[msumpter]

Another vote for XenForo, my previous employer ran several forums all using vBulletin. IMO the direction of vBulletin 5 was a big miss, and like other commenters have said, the core product manager/developer of vBulletin 3 & 4 left to start XenForo. There was some significant legal drama with the current owner of vBulletin suing XenForo for infringement & theft intellectual property. Those all have been dismissed in favor of XenForo (all from memory, probably worthwhile to search this out just in case).

I've seen some enthusiasm around Discourse but for users who are used to the look & feel of a traditional discussion forum or the workflow of vBulletin, it can be very jarring. I believe there was a decent community split when Ubuntu tried to retire their legacy vBulletin forum in favor of Discourse; I think they are still running both side by side with almost different groups using them. XenForo is a more natural progression from vBulletin and would be my platform of choice.

[h1d]

Discourse looks different from the traditional style but to my surprise I got used to using it quite fast and liking it much and not just about the usability but with its API, customizability, administration capability and mobile support. It shows when someone has experience creating stackoverflow.