Casa Labra, Called de Tetuan
Unlike the last visit, this is a busy time here (8 p.m.), with people spilling out onto the street with their tiny glasses of wine and beer, and tasty tapas. Elsewhere, being on the street might be unpleasant, but fortunately this one is pedestrianised, and thus free from the noisy traffic that clogs Madrid at night - when the worst of the traffic jams appear.
This is Casa Labra at its best, because however much it looks like a chaotic mess, the efficient waiters clear empty glasses and take orders for more drinks most conveniently. Even more importantly, the big trays of tapas are promptly refreshed when the run out, which is about every five minutes at the moment, so the tapas are always freshly cooked, hot at available. From my point of view, it is also nice that the waiters direct you to the least packed stretch of bar, which is relative when it is this busy, so it feels less bad to claim space there and is less intimidating to try the first time.
The only downside to this place is that it is good enough to raise the danger of staying all evening and having piles of deep-fried fish and croquettes for dinner. That's is the downside - no variety to the tapas, although perhaps that improves if you go and sit down for a meal in the room at the back.