AA has the best premium economy on USA to South America, but that is by default. AA has premium economy on most of its widebodies - only exception are the 767s which aren't used to SCL and EZE. DL and UA only have PE on some of their widebodies as of now - and who knows when they will start flying PE equipped planes to South America. Air Canada is also an option, but you have to connect in YYZ. Aerolineas Argentinas and LATAM don't offer PE.