Hmmmm. If you can do expo, then I do suggest that. Overall I'd say Chicago is the #1 pinball city. But, you can go to plenty of places and incorporate Pinball.
NYC
Vegas
Boston - You can go to Pelham, NH and visit Pinball Wizard Arcade which has the most pinballs next to the hall of fame
My vote goes to the expo in Chicago this year. Turning out to be a VERY interesting show. Chicago is closer than most of your options and it's a fun city.