Assuming that by "Spanish speaking country" you mean a country where Spanish is the official language, Cuba is the closest to Florida.
Cuba is the closest spanish speaking country.
Cuba.
Cuba. It is 90 miles from the coast of the United States.
Cuba
Well, luckily for you the Caribbean has a wide selection of Spanish speaking countries for you to choose from, just south of Florida. The closest Hispanophone (Spanish-speaking) country to Florida is none other than Cuba, just 90 miles as the crow flies from Key West. Of course, if you're not a fan of communism, there are other options. The second closest Hispanophone country is the Dominican Republic, but is around 900 miles from Florida. Also, if you don't feel particulary Caribbean, Mexico is always an option, only slightly further than the Dominican. If I were in Florida, I wouldn't travel beyond these three distance-wise.
Cuba
Mexico.
It depends on which country you live in. In the United states, the closest countries are Cuba and Mexico.
Cuba.
Apparently Mexico, Cuba and the Dominican Republic.
Mexico (borders the US) and Cuba (90 miles from the US).