wow tough choices there. All 3 are amazing but for simplicity Belieze was the easiest. The dive boat pulls right up to the ship and you literaly get right on the dive boat. You then goto the shop and sign papers and get right back on. Very little baggage handling. But for unforgetable experience Cozumel wins out and would be my primary choice.
Grand Cayman is not to be sold short though it has some spectacular dives and excellent walls to amaze people. :)