The SSME was a staged combustion but not full-staged combustion as the next SpaceX motor will be (Raptor). The SSME are special and specially efficent because

The SSME was a staged combustion but not full-staged combustion as the next SpaceX motor will be (Raptor). The SSME are special and specially efficent because they burn fuel-rich and have multible pre-burners. They were very highly optimized but that made them complex. Full-staged combustion has a better max performance for a given fuel and promises to be simpler in design.