Well, São Paulo (the Brazilian state) is a good place to start a company, it does have a large talent pool if you're willing to pay for the best (there are a lot of mediocre people working here), the majority of the best universities in the country are in the state, and it does have a decent infrastructure, though the government regulations are a mess for new business and not much people understands English as they should, São Paulo (the city) is expensive to live compared to other tech centres in the State (São Carlos and Campinas).

There are also other Brazilian states with a vibrant tech sector, the three Southern states, Rio de Janeiro and Pernambuco, but São Paulo alone probably have much more tech companies than they all combined.

Buenos Aires is a better city than São Paulo for me, the infrastructure is relatively better, but I think that the Argentinian State is even worse to business than the Brazilian, some of the same problems that you find in Brazil you'll also face in Argentina.

Santiago is like Buenos Aires and São Paulo, although a bit smaller, the Chilean State is much more business friendly than other South American governments and the living is cheaper than São Paulo or Buenos Aires, but I do not know much about the working conditions for tech companies there. But I do know that it's easier to get foreigners to work in Chile than in other South American countries, and their taxes are much smaller compared to Brazil or Argentina.

EDIT: In addition, there's a growing start up scene in São Paulo and Buenos Aires, once again I'm not sure about Santiago, it does not have a tech sector so developed like São Paulo or Buenos Aires, but I think the government there is probably trying to change that.