Lagrassa's is easily the best sandwich you can find in downtown Boston. I have actually disciplined myself to only eat there for special occasions, since I could not stop going. Everything from the Reuben to the pressed veggie and chicken melt, you just can't go wrong. The potato salad is sweet and delicious and the authentic New York half sour pickles are a perfect compliment to there deli sandwiches. Prices are a little higher then normal deli's in the area, but the quality is far superior. The sitting area gets crowded fast, so have a back up plan in mind if you are looking on eating there. They do have a call ahead system which is convenient. If you've never been you are missing out, and if you have had the pleasure then you all know what I'm talking about.
Pros: Best sandwiches, potato salad, location
Cons: Closed weekends, tough seeting