China is the first claimant to the region in the modern era when nation like Philippine and Vietnam were still colonies of the west. Especially compare to Philippine whose border is well defined under the Paris Treaty between Spain and the US that excludes it from any of the islands in the South China Sea, Chinese claim are much stronger.
Beyond that, there is the Japanese occupation, which by Treaty of Taipei with ROC it has relinquished all control over the islands in SCS. And further back, the Chinese-Vietnamese Boundary convention in 1887 signed with the French also confirmed Qing's ownership of the Spratly and Paracel islands.