Which is country is best to learn spanish? (Costa Rica, Panama, or Paraguay)?

This summer I will stay and live with a family in either Costa Rica, Panama, or Paraguay but I don't know where to go. My first choice would be Paraguay because it is in South America and I have never been there. I think Paraguay is quite unique because it is in the middle of South America and lots of the people are mestizos(90%). The only problem though is that they speak Guarani also. Would that be a problem for me? My goal is learn Spanish better. If I go to Paraguay it would be for 6 weeks. While in Costa Rica and Panama it will be 4 weeks. I think Panama looks cool also due to its history, monuments, and modern buildings. While Costa Rica is rich with rainforest. Which country is better?