I've used both PHPbb and VBulletin. Both excellent programs, but VBulletin's support and the number of add-ons/hacks (nearly all of which are free) is just astounding.

There are sites of enormous size that run VBulletin without a snag (GFY and GWW are two examples), and between that and the custom features (and wide availability of skins), VBulletin gets my vote... of course, your mileage may vary