I've used vBulletin 5, vBulletin 4, Xenforo 1, Xenforo 2, myBB, and phpBB. I've never used vBulletin 3 as it's very old but I've been around the block a few times. Never had any big sites with anything but vb5, and xf1 and xf2. And by big I mine not small. I've never actually had a huge site. Would be nice if my current site gets big. Wishing and hoping that happens one day.