I admin several forums and for them all we currently use vBulletin. This is a stable and well maintained software for running forums. There have been some issues in the recent past how they may or may not have treated customers with a change to the pricing structure. I have stuck with them and have no issues. I am happy to pay my way. What I am not happy about is the flourishing black market in nulled vBulletin. This is the version of the paid software that has had the licensing and encryption information removed and is used illegally. Obviously those with nulled vBulletin will get no support from the company.
POSTSCRIPT: Just added this entry at PodiaPaedia on this topic.