I finally understood what you were asking and was looking on the wiki page of Spanish football rivalries. Apparently there are three clasicos in Spain that serves as a wonderful answer: El Clasico (Real Madrid vs. Barcelona), El Viejo Clasico ("the other classic", Real Madrid vs. Athletic Bilbao), and Athletic Bilbao vs. Barcelona is also considered a clasico but it doesn't have a name for it. Basically all three of these clubs are among the oldest in Spain that have faced each other the most compared to all other meetings of other spanish clubs. El Clasico is the most played game in La Liga (followed by El Viejo clasico and Barca-Athletic clasico) and El Viejo clasico is the most played game in the Copa del rey