NodeBB

NodeBB is modern forum software written in Node.js with MongoDB and Redis. It has a modern look and built on a modern platform. The plugin system works great and it's very simple to add an remove plugins. Your options for customization range from CSS overrides to a completely custom look and feel. The instructions for setup and even scaling are quite straightforward as long as you have a little Linux or devops experience. The team has a philosophy of not pushing back on requests and complaints and they actually work with you to find the best solution to almost any issue. Many times it's just a quick bug fix (and the team is super fast at bug fixes).