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