Been to Argentina, Chile, Brazil, Colombia and Paraguay. All great (well, maybe not Paraguay), but Argentina is the best IMO. BA is an incredible city, Bariloche (skiing) and Mendoza (wine) are also fantastic. Not even been to Patagonia and that's supposed to be the prettiest/coolest part. Brazil is edgier with more impressive things to see - Iguassu, the Pantanal, the Amazon, Rio - but a little rougher and harder to get around.
My daughter studied in Peru and absolutely loved it. I'll go there soon, but can't speak to it at this point. It seems to have a hugely diverse number of things to do/see.