I recommend definitely Colombia, either in Bogota or Medellin which are the two biggest cities. Besides knowing a beautiful country, the Spanish language there is pure, people do not speak in English, so it is very easy to learn, speak and practice all the time. I have traveled all South America and I must say that Colombia is the country that handles a neutral Spanish most similar to the original Spanish language. The best of all is that Colombia is now a safe country, with super friendly and warm people, a place where you will feel at home.