The best South or Central American City

For as long as I´ve been in the US, everyone could only talk about US cities when discussing the best "American" holiday spots. But what about the rest of the Americas? South and Central America are bursting at the seems with quality fun holiday destinations. Its time to inform the country about the best spots and why. Whats your favourite? Whens the best time to go? What should be avoided?

If you agree or disagree with any of the posted cities, feel free to add on the post in questoin a relevant comment. Otherwise just enjoy the read!