A very interesting question and a very tough one to answer. On the one hand you feel like you've seen one cruise port, you've seen them all. Bars, beaches and jewelry stores. On the other hand each port of call has it's own unique flavor. I have a favorite thing about each port I have visited.
My favorite itinerary is the one out of Tampa that takes in Cozumel, Costa Maya, Belize and Roatan. If I had to narrow it down to one it would probably be the Cays off the coast of Belize.