Buenos Aires was by far my favourite city in South America!
The food, atmosphere and nightlife are everything its hyped to be. In terms of value for money, the food you get there is more expensive than plenty of other places in South America, but it is all much better quality. The nightlife is phenomenal, and you must visit a milonga (tango club), they are incredible. A lot of them offer lessons too! It's a beautiful city to just walk around and explore all the very different sections.
Insider Tip:Take USD with you and exchange it there for a very good rate.