Actually my favorite is beautiful white sand 7 mile beach on Grand Cayman Island. Then again, I love calm turquoise colored waters with visibility in 20 feet of water. Believe it or not, some of the worst beaches on earth, if you even want to call them beaches, are in the Florida Keys. My wife and I once took a road trip down there expecting to do some nice shore line snorkeling and it was awful, what a dissapointment.
Keys' waters are great for boating and fishing, not so much swimming... although there are some nice looking, small beaches tucked in here and there from Key Largo to Key West.