I stayed at the W South Beach hotel on its opening weekend and fell in love. The rooms are spacious studio apartments with ocean view balconies. It's on the cusp of North Beach, so it's more peaceful than hotels further south but walking distance to the action and nightlife. The hotel is right on the beach, and the hotel sets up chairs, umbrellas and towels for guests with poolside and oceanside bar service.
My sister's also a fan of the Westin Diplomat Resort & Spa in Hollywood, FL though I've never been.