As US citizen, does one still need Brazilian transit visa, if travelling to another South-American country (eg. Argentina) with stopover in any of Brazilian airports?
The US citizens are required to have expensive visas to Brazil in "reciprocation" for US visas for Brazilians. Even if there is a stopover, a transit visa is required.
As far as I know, that the only exception of this rule is São Paulo-Guarulhos Airport. If your stopover is there, and you don't leave airport (don't try to go trough customs), you don't need transit visa. But only in this one airport.
A friend of mine was travelling from Miami to Buenos Aires. He was misinformed by his airline and had stopover in Brasília. He was deported for not having a transit visa.