Boca vs. River has to be number one...there is no other country in the world where so many people are for two teams. I've read statistics that say 40% of the population are Boca fans and 32% are River fans, but having lived there I can say it's much, much higher....more like 60% for Boca and 30% for River.
Of course for me, the best is Independiente vs. Racing (Argentina's second biggest derby).
But obviously the most attractive is Barcelona vs. Real Madrid, the two best teams in the world in the best league. Now with the Galacticos 2.0, all the best players in the world are on the same pitch at once.
I bet some of the German rivalries are great, though I'm not familiar with them...Werder Bremen vs. Bayern Munich maybe?