The Sultanate State of Sulu is the legal owner of the Spratlys Archipelago but since the Sultan Kiram turnover his territorial and proprietary rights of the State to the Republic of the Philippines including the Palawan and the Spratlys in 1960's, then legally it belongs to the Philippines.
Under the UN International Laws of Sea, despite the opposition of maritime powers, the Philippines and four other states (Indonesia, Papua New Guinea, Fiji and Bahamas) got the approval in the UN Convention on the Law of the Sea held in Jamaica last December 10, 1982. They were qualified as archipelagic states.
The Spratlys is within Philippines proximity and connected in the Philippines' archipelago with just few kilometres from the province of Palawan of the Philippines, this boost the claim of the Philippines. Other claimants are not archipelagic states and they are far from beyond the limit of the UNCLOS of 200 nautical Miles Exclusive Economic Zone. The Philippines as an archipelagic country & within proximity, it counts merit in administering the Spratlys.
UN Convention - International Law of Sea (UNCLOS) bases
China argued that there is no word proximity mentioned in the UNCLOS so they insisted that they could claim the Spratlys. Though there is no "proximity" mentioned, the UNCLOS clearly explain the 200 Nautical Miles Exclusive Economic Zone (EEZ) which is more clear explanation than proximity. Proximity is just to say near but the measurement of 200 Nautical Miles from the base line of the Philippines is not just synonym to proximity but a clear measurement of distance.
Under the UNCLOS, it guarantees 200 Nautical Miles Exclusive economic Zone which most part of the Spratlys is just within 200 Nautical Miles Exclusive Economic Zone of the Philippines. This is the strongest bases of the Philippines to say "We owned the West Philippines Sea". Even China is a signatory in these and they even agree with this together with the United Nations.
The History as bases of claim
Back to Majapahit and Sri Vijaya Empire in year 7000 or 7th century (The ancient kingdom in Indonesia and Malay Archipelago) the territory is extended from the North Borneo, Palawan, and the Spratlys of the Philippines' territory.
The Sultanate State of Sulu was established during that regime which area includes Part of Mindanao (Cotabato, Lanao, Zamboanga Peninsula), Basilan, Sulu Sea, and Sulu.
During the 14 century or year 1400 the King /Sultan of Brunei give as gift to his cousin Sultan of Sultanate of Sulu the "North Borneo, Palawan and the Spratlys" for helping him to win a battle. The China recognized the Sultanate State of Sulu that includes the North Borneo, Palawan and the Spratlys archipelago.
When the Spain invaded the Philippines in year 1621, the sultanate state of Sulu remains un-conquered but portion of the territory was controlled by Spain including the whole Island of Mindanao, and Palawan.
When the Britain gives independence to Malaysia, North Borneo is under a lease agreement which is until now is recognized by Malaysia was illegally included reason why the Sultan of Sulu calling the Malaysian Governmet to stop controlling his land "North Borneo" (Sabah) which is also link to Spratlys.
The Sultan of Sulu turnover his territorial and proprietary rights of the State to the Republic of the Philippines including the Palawan and the Spratlys in 1960's.
The History, the UNCLOS law of Sea, the archipelagic states theory says "The Spratlys is belonged to the Philippines.
The Spratlys archipelago
At the moment they are administered by the Philippines, but some are claimed by Brunei, Taiwan, China, Vietnam, Malaysia and Indonesia.
They comprise less than five square kilometers of land area, spread over more than 400,000 square kilometers of sea. The Spratlys, as they are called, are part of the three archipelagos of...