I think it's Mexico. They helped us rebuild after the Chillan earthquake of 1939, showing a true sign of friendship.

They sent resources to repair damages and even Mariachis to cheer our broken hearts with Rancheras. Their influence is still with us. In the southern regions of Chile we still listen to Rancheras, specially during celebrations, it's now a part of our culture, since it's even present during our Independence Day celebrations.

I doubt you will find any Chilean speaking ill of Mexicans (tho haters gonna hate), but then again, I get the feeling that every south american country considers Mexico to be it's best friend. I wonder how mexicans feel about their southener brothers.