The ‘ideal’ constitution

What is the ideal constitution? No actually, what is the least horrible constitution? The ‘best’ political system is an entirely different subject. Assuming the political system is a constitutional state, what should the constitution be?
In case people question why this article is listed amongst the philosophical articles, even though it analyses and compares an abstract political system, I would remind them that this constituted philosophy for Plato and Aristotle, so I am going to say that it counts as such.