Indulging once a month isn't a big deal
One of the first places my Catalan friends took me when I came to live in Barcelona and where I still go to sometimes. The Bar Central is located on La Rambla, inside the Boqueria market, more precisely on stands 494-496 and is the ideal place for an informal and quick lunch after a long walk in the center of the city.
There are no tables there, just a small counter with several stools around, always busy with tens of people almost fighting to get some space to eat in such a picturesque place, especially during the rush hour (from 12 to 2.30 in the afternoon).
This is why I recommend being very patient and ready to wait a few minutes before you get a spot. The truth is it is worth it. The products are of the finest quality: fish and seafood mainly, although there is also meat and vegetables.