In my opinion, this is the most influential building in modern Barcelona. Locals used to call it "el supositorio" (the suppository) or even worse things, but it is now admired for its presence and versatility. Designed by Jean Nouvel and unveiled in 2005, it employs lights and glass in a way to allows the whole structure to change color, and it’s now one of the main attractions for architecture lovers—both visitors and locals.