Depends on what you like to do. We've sailed on Carnival Paradise three times. First time to Eastern: Nassau, San Juan, and St. Thomas. Second to Exotic Western: Belize, Roatan, Cozumel, and Grand Cayman. Just back yesterday from third to Exotic Eastern: Nassau, St. Thomas, and Dominican Republic. Also have been to Cozumel and Grand Cayman on Sensation.
Grand Cayman has great shopping and great water stuff. Swimming with Stingrays is a must-do if you're getting in the water.
Cozumel has shopping and good water stuff. We enjoyed visiting Chakanaab Park for water stuff on our own after shopping.
Belize: We took the Goff Caye Snorkeling excursion and enjoyed it a great deal. Others did Shark Ray Snorkeling -- that sounded more fun than ours. And, still others booked independent cave tubing with Reggie and most had a great time. Belize is definitely a water day.
Roatan: We took the ParrotBay Beach Party Excursion. It was mostly laying around a man-made cove ... relaxing, but not for everyone. As we headed out to this excursion on the tour bus, we asked the driver what the best thing was to do on the island. He said go to the West End. That evening we met up with some friends who had caught a cab on their own to the West End, where they shopped then got in the water for the day, and they loved it.
We loved all these islands ... a great itinerary!