My favorites are Cancun, Aruba and St. Thomas. It really depends on what you are looking for.

I thought the people on Aruba were the nicest, it had Casino's and is out of the hurricane belt, but the island itself is not beautiful and is windy. Cancun has beautiful beaches and plenty of activities, but has so many hotels and very Americanized. St. Thomas is the prettiest, but less night life than Cancun.