If you consider a worlwide survey, China is definitely the greatest one, before, during and after Rome.

That's the standard opening to point out this is a discussion about Western Civilization, but with China it is a tough call. A lot of things happened there and then elsewhere not all that long thereafter. Silk was not the only thing to come down that road. IMO, early China had a lot of indirect impact on European development. I would concur about them having the greatest ancient civilization if you consider other parts of the world.