I think Buenos Aires has the most soul, but that's just because you're seeing the remains of a wealthy society that basically just plummeted into oblivion.
São Paulo is one of the ugliest cities I've ever been to. I like some parts of Santiago, Buenos Aires and Curitiba, in southern Brazil.
The thing is, in most (not all) large South American cities, crime is rampant, lack of infrastructure is a huge problem and poverty is widespread. I really wouldn't want to live in any of the cities you mentioned in the OP. I mean, Santiago is nice, but I wouldn't live there. :P
That's not to say there aren't wonderful places and cities in the continent though.
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