I believe mybb is the best forum software out there atm, because of it's easy plugin system and how easy it is to configure. I've been using it for years and I probably won't be changing anytime soon. I even wrote an installation guide for it to help beginners out.
The "best" software would be really hard to determine... "best" commercial or "best" free forum software might be a little easier to narrow down but vb 3.x would be my favorite.. with xenForo looking at the future. I believe xf is going to grow with a solid core and I believe their community of webdev gurus will compare to the good vb days.