MyBB:
MyBB is a free forum software that provides the forum owner an extensive amount of abilities and features. There are two things that really grab my attention from all other forum softwares - the group system and most plug-ins. The group system is a necessity for forum owners whom look for constant activity. The way only administrators can make it official. It's a great addition. Neatly designed and focused on. A lot of the plug-ins are great. The can really change the whole MyBB software. And that's why I like it.

vBulletin:
vBulletin isn't perfect in my opinion. I also dislike the fact that it requires to be paid for when MyBB tops it. Everything a forum owner would ever look for is in MyBB. vBulletin's group system is crappy. Anyone can make an official group. There is no activity increased. And, it all seems so dead. The plug-ins for vBulletin don't change vBulletin at all. There are no goodies.

If you've read my review on both these forum softwares, you'd know I'd pick MyBB no matter what. MyBB tops all forum softwares - phpBB, vBulletin, XenForo, etc.