rogerd - 3:11 pm on Feb 15, 2010 (gmt 0)

vBulletin is highly scalable, and although my biggest forum is busy, has a solid revenue model, and could afford any commercial app, I am not contemplating a change. vBulletin is continuing to invest in the product, and the plugin system has made modifying it and periodically upgrading it a lot more straightforward.

That's not to say that other solutions couldn't do the job. Still, vB would be my recommendation. It's the most popular choice among "big boards."