I first went to La Ardosa several years ago on a Friday at lunchtime and was wowed by how good the…read morefood and the service were. As it was before the days of Yelp I wrote them a glowing report on the customer-feedback cards they have. Since then, I go whenever I'm in the area and never regret it.
In my opinion it's important to beat the crowd, because the main room is small and gets really crowded. So go before 2 for lunch and before 8 for raciones, beers, etc. Also, crouch down and go under the bar into the back room, because there you won't be on top of strangers and their cañas.
Some great dishes I've had there include the callos, the seared artichokes (in season!), the marinated dogfish, and the croquettes. All really good. The only drawback: €20/person for beers and raciones is above the going price, though I would recommend La Ardosa to anyone who loves Spanish food.