Boca v River, before River got relegated. Obssesses the whole continent when it's played... both have generally been contenders for Copa Libertadores while the Old Firm haven't had a proper go at the European Cup in nearky two decades. I don't think many people outside the British Isles, excluding Scottish/Irish diaspora, care much about Celtic-Rangers.
What about internationals, our own England v Scotland has to be up there along with Brazil v Argentina, Holland v Germany always intense too.