September/October is the end of the dry season in Peru, and a really nice time to be there in the mountains. Although it's the wet season in Venezuela, in Brasil it's largely dry, and the paved roads are fine regardless of weather. Argentina & Chile are wonderful in December/January, but Peru and Bolivia are well into the wet season between January and May.
Taking into account country preferences, I'd go down the west side via the amazing mountain roads of Peru & Bolivia, cross the Salar de Uyuni to San Pedro, cross over on one of the passes to northern Argentina, then south on Ruta 40, dipping into Chile as required. Back up on the east coast.