I have a biais for Argentina, thinking people are nicer and food is better. You can also go zigzag between the two countries over the Andes from Santiago. That's what I did. Don't miss the Valles Cachaquies in Salta province.
For the Darien crossing, indeed there is a lot of info here. There is an adventurous way from Buenaventura on the Pacific side, but you would miss a lot of Colombia if you go straight there, which is sad as it is in my top 3 country.