Why did G-d choose Israel to be His holy nation? Why didn’t He choose China, Brazil, Argentina, etc.? What makes Israel so special out of all the other countries?

This is a very good question. It's not because we were good or special or anything else that we were chosen by Hashem – it's the other way around, because we were chosen by Hashem we became special and with great responsibilities. We don't know why Hashem chose us and not the Chinese or any other nation. We know Avraham was extremely righteous, we know the other nations were not too good at the time but not because of that he was chosen, because even if we misbehave or even worship idols – we are still the chosen people only because Hashem said so. Sometimes we just don't know why Hashem does things we only know that that is what he wants.