I won't go into great detail, since my last overlong positive review (Franny's) got nixed on suspicion of shilling.
But for those (like me) that are continually dissatisfied with the cold-sandwich options in Brooklyn, my partner and I are recently thrilled with Zoila, a modest little sandwich counter on Hoyt between State and Atlantic (that's approximately Boerum Hill, I think, though it's also just part of the general Atlantic Ave area).
Excellent sandwiches - familiar but slightly original, expertly composed, neither overstuffed nor skimpy, with simple but high-quality ingredients. The "Venezuelan lemonade" (with honey) and the potato salad are very tasty. The two women who run it (seems to be mother-daughter) are super nice and the whole experience of getting food there is very pleasant. They have a little raised shelf with a few stools and a bench outside if you want to eat there, but it's more of a take-away place than a restaurant. They definitely deserve 'hound patronage.