Honestly, I hate phpBB with a passion. It suffers from the same affliction so many open source platforms are hobbled by: reliance on plugins for anything beyond barebones functionality. But those plugins are often of awful quality, and then cease to be maintained and break with updates to the platform.
I used to be an admin (as in, super-moderator, not a server admin) on a major Mac forum (60K+ registered users in its heyday), and after its start with UBB (a Perl-based forum that worked well with a few hundred users but didn't scale AT ALL), it moved to vBulletin, an excellent commercial product. It, too, supports plugins, but since it's actually got support and active development, things actually work. Being a Mac forum, there were quite a few web designers among the members, so some were paid to actually code a custom theme and a few custom customizations, which actually work well.
Alas, people seem to invariably reach for open source forum software nowadays, despite the fact that they require immense amounts of babying to keep working well. A false economy IMHO.