well, I think if we are talking strictly about efficiency, the intuitive answer would be: either a single government, or at least a government whose boundaries are set based on more practical and real rather than artificial foundations.
National identities, as complex as they are and as huge an improvement over religions as they are to serve for the above purpose, are still artificial constructs that in many ways create arbitrary boundaries, introduce and perpetuate differences that are not necessary to exist between people and in some cases have even been manipulated and used by governments for political reasons.