Alright, I was offended that IPB had as many votes as vB.
IPB is a much more, detailed, and easy to use forum software.
I've used it on plenty of sites, it's looks are phenomenal.

Although, I will give it to MyBB for being a more user friendly type of forum software.

I would have to go with MyBB. I've been using MyBB for a couple of years, and my popular forum, UG, was using MyBB's forum software. Not only is it extremely customizable, it's also completely free.

I couldn't see myself ever running a forum using IPB forum software, maybe vBulletin or XenForo, though.