If I had to choose one singular and authentic bar in Barcelona, I'll say "Mendizábal".
This is not a fancy bar, and that's what I love of Mendizábal. But it doesn't mean it isn't a cute one with a beautiful decoration and good prices.
They have a two very differentiated spaces: one privileged terrace in the city center, and the interior salon. Them both are awesome, and it's perfect because you can decide which one fits better with the situation.
They do tapas, sandwich, juices, burgers and veggi burgers.
In my opinion, they have the best vegetarian sandwich offer in town.
But the most important, you can leave the bar without eating their "patatas bravas"
This is the up & coming market of Barcelona. Happens every first weekend of each month.
There are free concerts, food trucks and little designer shops.