Concierge.com's insider take:
Though they tend to be smaller and on the narrow side, there are so many pretty beaches in Fiji, they are impossible to not find. Trying to identify the best beaches in Fiji is easy however–they are often already the location of a tony resort. Among the resort beaches, standouts include Vatulele Island off Viti Levu, Quamea Resort off Tavenui Island, Navutu Stars and Yasawa Island. In general, the best beaches are found in the Mamanuca and Yasawa Islands (thus, all the resorts concentrated there). Some of the most notable in the Mamanucas are on the smaller islands–Beachcomber, Bounty, and Castaway–and you can see these on a day trip from Nadi (see South Sea Cruises), and patronize the sometimes rowdy bars and restaurants at the resorts there. Mana Island is also a stop on South Sea Cruises and is a bit bigger and more upscale. The outer islands of Matamanoa, (next to Monuriki where the Tom Hanks' movie Cast Away was filmed), Tokoriki, and Vomo also have lovely beaches but are private island resorts.
The best beaches in the Yasawa Group, are on Waya Island and Natuvalo Bay on Naviti Island, and on Tavewa island and Matacawa Levu Islands' south side.