I have 3 favorites actually. I have to say the most beautiful beach spots are actually in the Bahamas, Paradise Island. The best is Paradise Beach for sure.
St. Barts is fantastic. It's classy yet fun and carefree. It is rather expensive there, but there's excellent shopping, good restaurants, beautiful hotels, and of course great scenery.
I have to say my all time favorite is in the Turks and Caios. It's just everything a caribbean island should be. The friendliest people, clean, gorgeous beaches and green everywhere...it's just paradise. Best snorkeling and scuba diving, and the prettiest sunsets you could ever see :)

Wow so much to comment on, great replies so far. I must say Sunnyboy your assessments are fantastic. I've been to just about all the islands everyone has mentioned and can add to what others have already posted.

St. Lucia is great, certainly lush, no real international type restaurants but more local Caribbean cuisine...which is great too, depends on what you are looking for.

Anguilla, I agree on the food. Beaches are great, can't go wrong with Anguilla.

St. Martin, good selection of restaurants but heavily touristed island.

Same with Barbados. Although Barbados has some interesting caves to explore that other islands don't have and about the best fresh water supply of any island.

Provo, Turks and Caicos. Not a huge variety of restaurants but good ones. Grace Bay beaches are fantastic and good snorkeling.

St. Barts. Tres chic and expensive. Great place, Caribbean cooking with a french flare. Very upscale and excellent restaurants.

Antigua. Its so-so, perhaps why its not talked about too often. Nothing about it really stands out but there's nothing bad about it. Nice beaches.

Nevis. Sunnyboy's description is perfect. Restaurants are local Caribbean cooking outside the few resorts. Wonderful place.

Aruba. Heavily touristed, nice beaches and great restaurants.

Some not mentioned:
Tobago. Lush, untouristed, great snorkeling and diving. Restaurants are generally more local Caribbean cuisine.

Bequia. Same as Tobago except not as good snorkeling and diving.

Bonaire. Flat, dry and not great beaches. Why go? The diving and snorkeling is excellent. Very very laid back place. Restaurants, its your typical international mix with some local places.