I have travelled to most large south american cities, and the one i was most impressed with was medellin in colombia. It is clean, people are very friendly, and the metro system is the best by far in south america.
The quality of food (especially fruit/veges) and selection of products in the supermarkets is as good or better than most western countries, with prices for most fruit and veges 1/2 the price or less of bsas, and most other stuff either similar in price or cheaper, sometimes alot cheaper. The things that stick out in my memory is the quality of the bread (not like the doughy crap they sell here), and cereals - great selection. Try to get a US$ 1.50 to 2.50 hot lunch (soup and main with juice) in BSAS - is the norm there.
Rent is 1/2 the price of here, and the apartments and furnishing are better.
The climate is mild, a nice 20c to 22c most days, and cool nights- they call it the city of the eternal spring.
There is plenty of green spaces, and if you want to get out to the country side for a break, there are plenty of parks, small towns etc on the cities doorstep.
The only downside for me was its not close to the coast.