I was born in the district of Sarrià, a neighborhood where the rest of the Barcelonians don't really come unless it's for a good reason. And if there is a tapas bar that is worth travelling for, it's this one.
Bar Tomàs is considered to have the best patatas bravas in Barcelona, and in the weekends there is often a line to get in.
The venue isn't anything special, and the waiters are known to be not very friendly - but once you accept that, it's part of the fun.
Order your patatas bravas and you'll get a small plate of freshly fried hand-cut potatoes with garlic aioli sauce and a red spicy oil. So good, I still go there from time to time, even if I don't leave in the area anymore.