Half Moon Cay by far!
Been to both and though Coco Cay is really nice, Half Moon Cay blows all the other Cruise ship Islands away. A word of caution, however... you will never want to leave and you'll try to figure out a way to stay there, just beautiful. I've been to all the "private islands" on each cruise line in the Caribbean. My top three in order of preference is.. 1) Half Moon Cay - Holland America & Carnival 2)Labadee - Royal Caribbean 3) Castaway Cay -Disney. As you can see, Coco Cay did not make my top three.