The form of government depends a lot on the country, its history, culture or homogeneity. For the EU the best system I see is a highly decentralised form of federalism, with small states which borders would be defined by linguistic-cultural boundaries.
For a homogenous country like Japan, federalism isn't necessary.
Representative democracy is the best choice so far, but I find that in too many cases it turns to be a oligarchy, as eligible politicians are too few and stay for too long. One think I like with having lots of small states in a country is that it increases democarcy, as more politicians are elected per inhabitant (it is also more costly for the taxpayers...) and they are closer to the people.