I, for one, think Playa d'en Bossa is one of the best beaches on the island. It's long, sandy (no rocks), no plant life in between your toes in the water, relatively clean, and the water's not shallow like a kiddy pool but not too deep.
All the other quieter beaches I've been to fail in one of those categories, even the ones I really like (rocky, too much seaweed, too shallow, dirty)
PdB's only downside is that it gets a bit crowded.
Bora Bora is great because they blast some great tunes and have bar and food service on the beach. Late afternoon, you can amble up to the bar and dance. Beach chairs and umbrellas cost 6-7 eur/day to rent, each.
Cala Jondal is another nice beach (aside from the pebbles along the coastline). Great restaurant there with beach bev/food service, playing some good chill-out music. You'll need reservations though.