vBulletin... I've yet to see anything that comes anywhere close in overall quality!

and the new 3.5 version will blow everything else out of the water now that they've finally used AJAX in it... soooooooo much faster!

and i agree with Ryan about not getting off the ground... I honestly don't believe I've seen any other BIG BB's out there? I know every BB with over 50,000 members are always vB's :)