Dominica (ross)- is about as third world as you can get. You will not have clean water, will have power outages, no hot water.
Saba- Island is about 5 square miles, the total population is about 1200, including the med school. Very third world and primative island.
St. George (grenada)- Much nicer than Saba or dominica. not exactly first world country but not third. Only problem with Grenada is the crime rate is very high. Almost 3-3.5 times more than the US.
St. Marteen- Very nice island, big tourist place. not quite the US but much much closer than any of the other island.
If your looking for the island that is most like the US then look into St. Matthews on Grand Cayman. Cayman is very close to being like a really slow paced florida.