If you're looking for a PIP which the most people to attend I think you'd have to look at US cities with the most air service, like Chicago, Atlanta, New York, etc (ie top 20 airports).

I guess whoever organizes this event will have to decide somewhat between quality and quantity .. since all sorts of bums (including me ) might show up if it's a major area easy to get to, while more dedicated FTers would show up at a more interesting but somewhat out of the way location.