Rainbow Haven is great but it's like the "The Dome/Cheers" of beaches so expect to see a lot of young guys and girls wearing next to nothing...which really isn't a bad thing :) They can be a little obnoxious sometimes so extra effort is sometimes required to find a spot further down on the beach. Still a great beach though.
I prefer Crystal Cresent myself. Often mistaken for being a nude only beach it's not. There's three decent sized beaches there and the third beach is the nude one if you are feeling adventurous ;)
Martinique is a close second. Breath taking beach great for long walks and surfing as the water is quite rough there.