I probably say that the Colombian league is the strongest and the most competitive. There are pretty good teams in the other leagues but tend to weaker as a whole.
I can't remember the last time a team other than Peñarol or Nacional in Uruguay. Olimpia, cero and Libertad dominate in Paraguay. Colo Colo, U de Chile, Catolica and Cobreloa rule in Chile and the same goes for the other countries. Just a handfull of teams have a shot at winning the title.