Where is the ultimate spot to hit up in Massachusetts if you're looking for the best sub sandwich? Where is the Best Sub Sandwich Store in Massachusetts? Specifically, in Boston's South End, at Billy's Sub Shop is where you'll find the best sub sandwich store in Massachusetts. Here's what 'Love Food' had to say about what makes this spot the best sub sandwich store in Massachusetts:Billy’s Sub Shop is a favorite in Boston’s South End and beyond. If you find yourself in Boston's South End during this Fall, you might want to head to 57 Berkeley St. That's where you'll find the best sub sandwich shop in Massachusetts.