The Argentinian junta was one of the worst regimes to rule anywhere on that continent, but got far less media exposure here than say Pinochet's Chile, because of the spectacular coup d'etat in 1973 and subsequent murder of Allende. It was during a period where military regimes were the norm rather than the exception in South and Central America. Brazil, Uruguay, Chile, Argentina, Peru, Venezuela, they all had bouts of military dictatorship and totalitarian and authoritarian rule for decades of the 20th century.
One of the bloodiest was Videla's junta. I'd say it's about the only good thing about Thatcher; the Argentinian defeat in the Falklands War got the junta out.