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I'd say it is a close call.
On the one hand, the 70's was the decade of Operation Condor, when six South American military dictatorships (principally Chile and Argentina, but also Bolivia, Brazil, Paraguay and Uruguay) assisted each other in hunting down dissidents both in each others' countries and further afield. During this time, Chile was alleged to be rivalled only by Maoist China and the Khmer Rouge's Cambodia in having the world's worst record for torture, Uruguay had the highest number of political prisoners in the world per capita, while Argentina led the way in terms of killings and disappearances.
On the other hand, the 80's saw the emergence of civil wars and dictatorships across Central America, as well as that of brutal drug cartels and guerrilla groups in Colombia and Peru, who insofar as they had a presence before this time, it was on a much smaller scale. The PRI in Mexico was also clinging on to power by ever-more dubious means, such as the 1982 Corpus Christi massacre and the 1988 presidential election where, just as the opposition candidate looked like winning, the vote-counting system 'mysteriously' broke down. Even Venezuela, still regarded as a relative success story, was increasingly worlds away from its glory days of the 70's.
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