This forum uses IPB. I'm guessing the owner prefers it over other forum software.
I personally like xenForo. I jumped ship from vBulletin ages ago back when they had version 4 available. 5 was a complete mess from the reviews/opinions I've read. I'm so happy I did not stay with them. If xenForo was never made, I might have went with IPB, phpBB or MyBB. Probably IPB...
So out of all the free and paid forum software available today, which is your favorite and why? Please answer as an admin and a regular forum member.