What's the best place to eat dinner in Madrid?

Check out el Shirimiri, there are two, but as someone suggested this is best in La Latina. For super expensive you can also try Jockey, Zalacain, el Bodegon, Las Cuatro Estaciones and Saint Celony...

Fanegas 29, Lateral, Casa Camu are amongst my favorites. Try the La Latina and Chueca neighborhoods, they have some great restaurants!!

For a romantic atmosphere and good food I would also recommend La Capilla de la Bolsa. Some excellent suggestions here though.

Viridiana http://www.restauranteviridiana.com/, an experience that will change the way you think about food. A must at least once in life.

Gula, Gula (or gluttony, gluttony) for the best vegetarian buffet with live drag queen show, flamboyant servers and great ambiance.

DIVERXO is the absolute best.

If you like to have diner on a very nice place, with decent food, great presentation and specially a at a great affordable price, you can go to Bazzar (http://www.tripadvisor.es/Restaurant_Review-g187514-d777973-Reviews-Bazaar-Madrid.html). The only problem ios that they don't accept any reservations..but if you are willing to wait for about 30mins its a good choice.

Cool tapas restaurant: El Lateral, Gabinoteca and Juana la Loca. Fashion: Tomate and Le Garage Prestigious and expensive: Casa Lucio and Horcher Contemporary (very special&romantic): Assiana

I love 'The Kitchen' (http://www.lakitchen.es), near Cibeles and Chueca. It is a very nice place, perfect location, not expensive.