I use XenForo and see it as the natural successor to vBulletin. Community morale is high, the code is clean, the team is first rate and there is a wide range of available addons. I've considered NodeBB and Discourse for future projects.
richard_millington:
I think one of the bigger challenges in picking a platform is looking at which platforms have the best future.
This was definitely vital in our decision process as well. For evaluating future prospects of products in the "commercial forum script" category, I find the DigitalPoint Cookie Search tool to be indispensible:
https://tools.digitalpoint.com/cookie-search
The four biggest commercial forum scripts are tracked: Woltlab Burning Board, Invision Power (IPB; mentioned above), vBulletin (vB; mentioned above disparagingly by several above), and XenForo (the new kid on the block, also mentioned above).
Particularly helpful is the following graph that tracks migrations between the four products. It is regularly updated and shows a stunning trend toward XF and a somewhat weaker trend away from vB
As a very happy XF customer, I can see why people are switching.