There is something that I truely love about phpBB. Of course it has some early 2000s touch to it and of course there are a lot of things you could change, but the system in general has survived for a reason: It simply does the job it was meant to do. Even the current port to mobile is nice IMHO! Also it is part of the ecosystem I grew up with. I can not think of anything much more streamlined in terms of running a forum. To me that is essentially because it is all about communication and flow of information.