My favorite port is Castaway Cay, but I guess that's not what you're asking.
I'd rank the standard Magic ports like this:
St. Thomas
Grand Cayman
St. Maarten
Key West
Cozumel
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I like St. Thomas the best because you can also head over to St. John while you're there, I like downtown Charlotte Amalie for wandering or shopping, and I like the many different beaches there.
Grand Cayman
St. Thomas
Key West
St. Martin
Cozumel
I loved snorkeling with the Stingrays in Grand Cayman and the town seemed nice for shopping etc.
St. Thomas is great! I've been to St. John long ago and would love to go again some day. In Dec. we plan on spending the day at Coral World and Coki Beach.
I don't know if there's any one thing. I liked 7-mile beach, and the stingray excursion reminded me of the one I did in Tahiti. The overall mood on the island is very relaxing, I find.
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