South America is vast and amazingly varied - just think of the contrasts between Amazonian rain forest, the Atacama desert (the world's driest, the high Andes and the glaciers of the southern tip. Then there are the cities, ranging from sophisticated, modern, European Buenos Aires to the almost totally indiginous atmosphere of Quito or Lima, with their wonderful Colonial architecture. I would suggest you spend a lot of time going through the Lonely Planet guide to Sth America, before deciding. But if you pressed me for one choice, I would say Peru (but would probably change my mind 5 mins later!)