Without a doubt, the Parish Cafe is the best place to go in Boston for a sandwich. Chefs from the best restaurants (le Espalier, Olives, etc) have each contributed one sandwich recipe. The sandwich options range from the ordinary to the extraordinary, and are always incredibly good.
The Parish is also a great place to grab an afterwork beer. It doesn't get the happy hour crowd (a good thing), and generally attracts a solid, relaxed clientele. Definitely a great place all around.