I have two forums and two forum softwares. One is running on Xenforo which is my favourite and the other one is on VBulletin 5. I am planning for the third forum say next month and this time I would be choosing mybb.
At my current forum I am using myBB. The main reason is because it is a freely available software but also because it is a great software to create a forum with. It has many great plugins that can expand the functionality of your forum, beautiful themes that can be customized to what you want it to look like and has amazing support too from both the official myBB forums and other areas on the internet