House of Botin. Sobrino de Botin. Restaurant Botin. Whatever you call it. Google "Botin" and "Madrid" to find reviews. Superb dining experience, especially if you can get the basement. Reservations necessary.
There is a famous scene in Hemingway re this restaurant, I think at the ending of The Sun Also Rises, in which Hemingway states that they have the best suckling pig anywhere. I don't agree with that, but I would wholeheartedly recommend the chicken with vegetables. Much, much better than it sounds, especially with sangria.
There is also a hot chocolate and churros place somewhere, very popular for late night visits, but I didn't visit and don't know the name. Maybe some other poster will recognize it and be able to provide the name.
As to food in Spain generally, maybe a matter of tastes. I'm generally a sandwich person, very easy to please--just slap the meat or cheese between two slices of bread, please--but even then, it didn't really work for me.
Anyhow, Madrid is glorious. Spain is glorious. Hope you have a wonderful visit!