Lets be honest with Anna, there are no fantastic beaches on AC. Some are just better than some others. I think we have all been to beaches on other island that deserve the desingation:deluxe or luxury. They don't exist on AC. Yes Banyan Bay is very nice and my wife and I stopped while biking and had no problem using it to take a quick swim and cool off.
Sir Issac, I think your being a local may have had you giving out such high beach ratings. Let's face it nowhere will you find any places highly touting the beaches on web sights.
I don't think I would base where I stayed just because of the beach. You will most likely spend quite a bit of time roamimg all over the island anyway. Walking distance to town is important to some, lots of restaurants and it is here where you will experience the local flavors.