The worst restaurant I have ever been to in Madrid has to be Balboa Social Club, General Pardiñas, Madrid. The place is in an upmarket neighbourhood and from the outside looks like a decent enough place. Less could be further from the truth.
This has been a while ago so I can't really remember the details, all I remember is that the meat I ordered was like leather and when sent back replaced by something equally bad. The waiter was a dumb fuck and wanted to argue over the fact whether we ordered sparkling water or still water. Still water, he brought sparkling.
The menu was a system of combining red and green dots to get a specially priced 3 course meal, which should have been a warning. Why though? I wonder if the people who work there have ever tried the food there or if they go to McDonald's after work.