El Cambalache Horno Argentino (Calle Del Espirtu Santo, 28) is a very lovely and ridiculously delicious take-away joint in Malasaña, which serves a variety of quick-eats including empanadas and calzone-esque pizzas. With it's fusion of Argentinian and Italian cooking, try the beef empanadas (sometimes simply done or with a sugar glazed pastry with raisins) and a hefty portion of the cheese, tomato and rocket folded-pizza. This is very much a hole-in-the-wall and a good stopover if you're investigating the surrounding vintage shops and quirky cafes. This place closes quite early, relative to the closing times in Spain, so ensure to try this place during the day rather than trying to stumble into it after a night out.
If you're wanting a modern take on the traditional (and sometimes mundane) tapas dishes strewn all over Madrid, La Musa (Calle Costanilla de San Andrés, 12) is a crowd pleaser. Personally, I prefer the La Latina branch compared to the alternative branch found in the barrio Malasaña (Calle de Manuela Malasaña, 18); the restaurant and bar spaces, complete with football and ping pong table, are separated over two floors creating a more atmospheric setting. More importantly, they have a terraza to don your must-have Ray Ban sunglasses and enjoy a cold beer or strong gin and tonic. The Degustación de Tapas menu for two is a helpful choice if you're on the more indecisive side of eating, but from my experience, order La Bomba (a very tasty stuffed potato dish with a variety of dips), Empanadillas Japonesa (similar to the Chinese jaozi or meat dumplings) and the Brownie de Caramelo (a brownie so rich it'll turn your stomach, in a good way). This is on the more pricey side of tapas dining with dishes ranging from 2.80€ to 11.50€ , but it's worth the investment if you're wanting to veer away from the usual assortment of tortilla on bread, ham on bread and cheese on bread.
For a 24 hour food place with reasonably priced coffees and pitas, Greek and Shop (Calle Corredera Alta de San Pablo, 9) is a great choice for all times and days of the week. The disadvantages of living in Madrid are the opening and closing times: you're constantly chasing shops, supermarkets and restaurants as they open and close whenever they please. You'll find that restaurants and shops are often closed even when they're advertised to be open. Greek and Shop is a failsafe, even during the early hours of the morning, with its middle-eastern themed menu and mini-store. My personal favourites are the falafel pita, which you can customise to your palette, and the folded flat bread of mozzarella, tomato and basil. If you're an early riser, you can also find a variety of Greek and continental pastries to take away or eat-in with your café solo.
If you're craving Neapolitan style pizza and a cosy little eatery, La Luna Rossa (Calle de San Bernardo, 24) is probably the best pizza you'll find in Madrid. Recommended by Italians, it offers a simple yet pretty delicious menu of pasta, salads, desserts and, of course, pizza. The drinks aren't over-priced, which is a rarity in certain restaurants that aren't Spanish, and if you go during the Christmas season, you'll see some outrageous Christmas decorations. When I first went, I ordered the standard but ridiculously gooey buffalo mozarella margerita pizza, which was around 9€. Pizzas range from 8 to 15€, which can be considered a steep investment, but worth the price if you want a good pizza or to escape the hipster or 'la moda' scene of Malasaña.
Ojalá (Calle de San Andrés, 1) isn't here because of its quality of food; the burgers were fine and the service was reasonable. However, upon arrival to the restaurant, we thought we'd be sitting on the first floor where the majority of the clientele were happily drinking their wine and picking at their tapas. Instead we were led to a basement beach complete with sandy floors and extremely tiny wooden benches and tables. I understand the concept: cool and hip eating spot that brings the beach to central Spain. Practically, however, it doesn't work. We awkwardly perched-slash-squatted in a picnic area where the tables faced each other, resulting in you making awkward eye contact with equally awkward strangers whilst tucking into your burger. It also didn't help that we were sitting next to a couple who were getting all too familiar with each others' mouths... Be warned.
A very much raved about but surprisingly underwhelming tourist spectacle is Mercado de San Miguel (Plaza San Miguel). Yes, it is a beautiful building and located in the heart of Madrid, but unfortunately it lacks character and has given in to tourist exploitation. Even though the tapas are immaculately presented, the prices are extortionate for both drinks and food. A tiny serving of tapa can set you back 3€, when the same tapa would come free with a drink in another bar. It's worth a visit as it's situated near Plaza de Mayor, but I'd suggest branching out of central Puerta del Sol for eating and drinking.
...and the Darn Right Horrendous
For incredibly cheap eats and terrible quality, 100 Montaditos, which can be found on every street corner, serves watered down beer and awful sandwiches in bulk. That is all.