Oh wow, this thread needs to get back on track and away from this political debate...
Argentine had one of the best economies right up until the Great Depression. Problem is with Argentina, Chile and Brazil was that they were export based economies. But the specific changes to make this happen is beyond me.
More colonization would not be an answer, even if it were 'beneficial' British colonization. Just because a nation was colonized by the British does not give it an advantage. Belize is not better off than Costa Rica. Jamaica is not better off than the Dominican Republic. Every nation is different from every other and has unique events, lands, resources etc that causes different events.
Argentina, mostly beef and other food stuffs, saw prices drop with the economy and floundered. So, the way to fix this, roughly, is to have earlier economic reforms and industrialization, move towards production of some different goods, become more independently sufficient.
Brazil was based on coffee and milk. Coffee was hurt in starting with WWI since it is less essential than food and so Europe, and America, stopped buying that first. Brazil at the time was controlled by plantation owners who were clinging to power in the face of a growing industrial middle class, partly growing due to European immigrants, usually with more liberal slants than the local populations. So perhaps have a greater amount of immigrants somehow that cause earlier progressive reforms before economic downfall?
Chile was nitrates through and through. British blockade of Germany spurred the demand for alternate ways of creating explosives synthetically, making Chile's monopoly on natural deposits worthless.
Couple other benefits could be attained from avoiding wars. War of the Triple Alliance comes to mind which destroyed Paraguay for decades upon decades, and Brazil ran up a huge debt which took decades to pay off which severely hurt their potential for growth.
The War of the Pacific also did severe damage. Bolivia was cut-off from the sea by Chile and their relations still haven't thawed since this war. In Chile, British control of the nitrates market increased noticeably. Peru was seriously damaged by the war as well.
Colombia's problem has been civil war and strife that it cannot escape many reasons, many of which seem to economic and class based, but I do not want to start a different political argument.
The Caribbean was obviously affected by constant US intervention to protect their own economic interests and domination, at the cost of the local populations. This was taken even further during the Cold War because of their 'containment' policy.
Basically, South America was hurt by their own internal bickering. Rather than cooperation, their conflicts over resources and land led to all being weaker for it. A parallel can be drawn from South American history to this thread - the bickering that has so far been the dominating current of this thread has severely hurt its potential.