The most beautiful beach I've ever seen while traveling - and I've seen quite a few beaches along the ride -can be found in Varadero, on Cuba.
Varadero is located in the boondocks of Matanzas and is approximately 90 miles / 145 kilometers far out from Havana. I've visited Cuba in 2015, together with my best friend. Despite the fact that we got told that Varadero has nothing to do with the life on Cuba and that we wouldn't find any typical Cuban lifestyle there, we were still convinced: we had to see Varadero and we hadn't been disappointed.
Of course, there are more hotels in Varadero than anywhere else on Cuba but that's it. The locals are also driving old-timers around here and everything's relaxed.
I've found a pretty quiet hotel for us, close to the top of the peninsula, without the party crowd. The beach was about five minutes walk from the hotel and was open to the public, so no annoying animation program there either. Just everything the heart desires.
Unbelievably glassy, calm and insanely turquoise water, green palm trees and sand so white as you only know it out of a travel brochure. No crowds and we felt like we're in paradise.
But I cannot guarantee that it'll stay like this since the relationship between Cuba and the USA has calmed down now. I'm not saying that this is a bad matter but things in Cuba are about to change, so use your time and make your way to Cuba, it's a journey that's worth it.
By the way, we took a ride on the local bus, called Víazul, from Havana to central Varadero. That works just perfectly fine, isn't too expensive ( single ticket for 10 Dollars) and takes about 2 1/2 hours. And yes, our bus was equipped with an AC ;)