Yes it is hard to pick just one, so I'll pick 5. Feel free to pick more than one if you like.
I would have to say that Half Moon Cay, Holland America's private stop in the Bahamas on Cat Island is one my favorites. The water is so clear. The beach is shaped just like a half moon and the water is so calm. If you ever get a chance to book a cruise that goes to this stop, book it. I believe Carnival now shares with HAL. I really enjoyed swaying in the hammocks, imagined myself living here and didn't want to leave. Of all the "private islands" in the Caribbean that cruise ships call on, I give the best award to this one, though to be fair I've never been on NCL's. If there is a Heaven on Earth, it would be here, weather permitting of course.
Another more recent favorite would be White Bay on Jost Van Dyke, BVI. Went here on a day sail from St. John. White Bay has the most beautiful water and like Trunk Bay and other pristine beaches on most of the Virgin Islands, you can look out and see a other islands in the close distance. I visited the famous Soggy Dollar bar and we hit the hammocks under the swaying palm trees on the white talcum soft beach while sampling the notable Pain Killer Rum drinks that the Soggy Dollar made famous. There is no dock at this beach so you have to anchor and swim ashore with your soggy dollars, hence the name. Some day I'm going to go back there and book a cabin at the small hotel on property.
Like other posters Trunk Bay on St. John, USVI is also one of my top picks because of the awesome view, crystaline water and the snorkeling. I'll give another vote for Mourne Rouge Beach on Grenada and another beach we visited on an island somewhere off the coast of Venezuela. I never did get the name of it. It was one of three islands we stopped at on an excursion. On the boat I spotted a whale which surfaced so close to the boat, it startled me and I fell off my chair. I think I was drinking something that day too. The day was perfect and the sand was so blindingly white. The setting was the epitome of the Caribbean beach dream, just perfect! Natives climbed the palm trees that hung over the edges of the beach to cut coconuts to sell us. I was in envy of them because they got to stay here. While on Jost Van Dyke I offered a resident my home near Orlando in trade for his. He agreed but then I wondered how I could make a living here. My first thought would be to open a beach bar but I think they might have enough bars already, open for ideas.
The Grand Anse beach in Grenada is a beauty as well. It's much more commercialized now than when I first went there, but most of the islands are. You can't keep heaven a secret. As far as making a living, you can buy a catamaran and take out snorkeling trips. I've been trying to talk my hubby into it. He keeps whining about having to get insurance, licenses, fees, etc. Plus we're not spring chickens any more. I also suggested opening up a beach rental business for chairs, loungers, umbrellas, maybe even hammocks. Again, he whines about licenses, fees, etc.
Another couple of favorites are Horseshoe Beach in Bermuda and 7 mile beach in Grand Cayman.
We went to Trunk Bay. As expected, the water was beautiful, weather was fantastic, and we had little fishies trying to swim up my brother's trunks. But that wasn't the best part of the day. While tanning on the beach with my sister, we're approached by some semi-intoxicated vacationer. He attempts to be suave in the following manner:
Him: Heyyyyy ladieeessss.....so, me and my friends got a boat nearby. We're going to party it up on another island later. We got lots of rum too. You in?
My sister looks in the water and starts laughing.
Him: What's so funny?
Me: You see that asian guy in the water glaring at you? That's our dad.
Him: Well, he'll let you go right?
Me: Our 15-years-of-kung-fu-experience-and-super-overprotective dad.
Him: Oh. Right. Later.
My dad later thought the entire situation hilarious and claimed that he simply had sun in his eyes and he hadn't worried because we have boyfriends trained in tae kwon doe and judo.
So that would be the best day we've had in the beach so far. Well that and my mom [tanyababanya] trying to snorkel when she clearly cannot swim. She put her head in the water and instantly brought it back up wondering why she got water in her mouth and wasn't the tube supposed to help? "Yes, but not when you put the whole thing underwater, mom."
Dickenson Bay in Antigua...took a taxi to Tony's, rented a thatched umbrella and chairs, grabbed a strawberry daiquiri and a burger, and just laid back and relaxed. Most beautiful beach I've ever seen!
Booked on Carnival Freedom, Feb. 2012 (Cozumel, Costa Rica, Panama)
NCL Gem, Sept. 2011
NCL Jewel, Feb. 2011
NCL Dawn, Sept. 2010
NCL Spirit, Feb. 2010
NCL Gem, March 2009
RCCL Explorer of the Seas, Feb. 2008
Carnival Miracle, May 2007
Carnival Conquest, April 2006
Caribbean Princess, Feb. 2005