Which are your favorite cities in South America, and why?

I read there's a fantastic city. The temperature is 70-80 degrees F year round. The locals are friendly and love the nightlife. It's a big city with big city conveniences with plenty of nature close by. There's mountains if you like the mountains, and a river if you like the water. Everyone is very laid back and casual. Cost of living is low.

Medellín, Columbia.