According to the Catholic Church, what is the best form of civil government?
I am looking for references to Church documents.
The Church does not support any particular form of civil government as being best.
The Church is impartial on this subject as long as the rights of the Church, the faithful and the natural law are preserved.
Pope Leo XIII teaches the following:
Lastly, there remain those who, while they do not approve the separation of Church and State, think nevertheless that the Church ought to adapt herself to the times and conform to what is required by the modern system of government. Such an opinion is sound, if it is to be understood of some equitable adjustment consistent with truth and justice; in so far, namely, that the Church, in the hope of some great good, may show herself indulgent, and may conform to the times in so far as her sacred office permits. But it is not so in regard to practices and doctrines which a perversion of morals and a warped judgment have unlawfully introduced. Religion, truth, and justice must ever be maintained; and, as God has intrusted these great and sacred matters to her office as to dissemble in regard to what is false or unjust, or to connive at what is hurtful to religion.
From what has been said it follows that it is quite unlawful to demand, to defend, or to grant unconditional freedom of thought, of speech, or writing, or of worship, as if these were so many rights given by nature to man. For, if nature had really granted them, it would be lawful to refuse obedience to God, and there would be no restraint on human liberty. It likewise follows that freedom in these things may be tolerated wherever there is just cause, but only with such moderation as will prevent its degenerating into license and excess. And, where such liberties are in use, men should employ them in doing good, and should estimate them as the Church does; for liberty is to be regarded as legitimate in so far only as it affords greater facility for doing good, but no farther.
Whenever there exists, or there is reason to fear, an unjust oppression of the people on the one hand, or a deprivation of the liberty of the Church on the other, it is lawful to seek for such a change of government as will bring about due liberty of action. In such case, an excessive and vicious liberty is not sought, but only some relief, for the common welfare, in order that, while license for evil is allowed by the State, the power of doing good may not be hindered.
Again, it is not of itself wrong to prefer a democratic form of government, if only the Catholic doctrine be maintained as to the origin and exercise of power. Of the various forms of government, the Church does not reject any that are fitted to procure the welfare of the subject; she wishes only - and this nature itself requires - that they should be constituted without involving wrong to any one, and especially without violating the rights of the Church.
Unless it be otherwise determined, by reason of some exceptional condition of things, it is expedient to take part in the administration of public affairs. And the Church approves of every one devoting his services to the common good, and doing all that he can for the defense, preservation, and prosperity of his country. - Libertas
The same Sovereign Pontiff states:
The right to rule is not necessarily, however, bound up with any special mode of government. It may take this or that form, provided only that it be of a nature of the government, rulers must ever bear in mind that God is the paramount ruler of the world, and must set Him before themselves as their exemplar and law in the administration of the State. For, in things visible God has fashioned secondary causes, in which His divine action can in some wise be discerned, leading up to the end to which the course of the world is ever tending. In like manner, in civil society, God has always willed that there should be a ruling authority, and that they who are invested with it should reflect the divine power and providence in some measure over the human race.
They, therefore, who rule should rule with evenhanded justice, not as masters, but rather as fathers, for the rule of God over man is most just, and is tempered always with a father's kindness. Government should, moreover, be administered for the well-being of the citizens, because they who govern others possess authority solely for the welfare of the State. Furthermore, the civil power must not be subservient to the advantage of any one individual or of some few persons, inasmuch as it was established for the common good of all. But, if those who are in authority rule unjustly, if they govern overbearingly or arrogantly, and if their measures prove hurtful to the people, they must remember that the Almighty will one day bring them to account, the more strictly in proportion to the sacredness of their office and preeminence of their dignity. "The mighty shall be mightily tormented."(2) Then, truly, will the majesty of the law meet with the dutiful and willing homage of the people, when they are convinced that their rulers hold authority from God, and feel that it is a matter of justice and duty to obey them, and to show them reverence and fealty, united to a love not unlike that which children show their parents. "Let every soul be subject to higher powers."(3) To despise legitimate authority, in whomsoever vested, is unlawful, as a rebellion against the divine will, and whoever resists that, rushes willfully to destruction. "He that resisteth the power resisteth the ordinance of God, and they that resist, purchase to themselves damnation."(4) To cast aside obedience, and by popular violence to incite to revolt, is therefore treason, not against man only, but against God. - Immortale Dei
In his allocution to the Consistory of June 17, 1793, Quare Lacrymæ,* commenting on King Louis XVI's death in January that year, Pope Pius VI said monarchy is the best form of government (“praestantioris monarchici regiminis forma”).
St. Thomas Aquinas's last word on the best form of government is in his Summa Theologica I-II q. 105 a. 1 c.:
Accordingly, the best form of government is in a state or kingdom, where one is given the power to preside over all; while under him are others having governing powers: and yet a government of this kind is shared by all, both because all are eligible to govern, and because the rules are chosen by all. For this is the best form of polity, being partly kingdom, since there is one at the head of all; partly aristocracy, in so far as a number of persons are set in authority; partly democracy, i.e. government by the people, in so far as the rulers can be chosen from the people, and the people have the right to choose their rulers.
Unde optima ordinatio principum est in aliqua civitate vel regno, in qua unus praeficitur secundum virtutem qui omnibus praesit; et sub ipso sunt aliqui principantes secundum virtutem; et tamen talis principatus ad omnes pertinet, tum quia ex omnibus eligi possunt, tum quia etiam ab omnibus eliguntur. Talis enim est optima politia, bene commixta ex regno, inquantum unus praeest; et aristocratia, inquantum multi principantur secundum virtutem; et ex democratia, idest potestate populi, inquantum ex popularibus possunt eligi principes, et ad populum pertinet electio principum.
Similarly, the great doctor of ecclesiology St. Robert Bellarmine, S.J. asks in De Romano Pontifice (On the Roman Pontiff) ch. 1: "What Might be the Best System of Government?" He argues in ch. 4 "That Without the Circumstances of this World, Simple Monarchy Would Absolutely and Simply Excel," but, in ch. 3, "That Monarchy Mixed with Aristocracy and Democracy, Should be More Useful in this Life".
It is dogma that the Church's government is a monarchy; it's constituted under one head having supreme power, the Pope.
cf. Vatican I's Pastor Æternus on papal primacy
Pope St. Pius X, letter Ex quo (26 Dec. 1910), Denzinger 2147a (3555):
with no less falsity, one is invited to believe that the Catholic Church was not in the earliest days a sovereignty of one person, that is a monarchy
[Latin original:] deinde non minori falsitate inicitur persuasio, Ecclesiam catholicam non fuisse primis sæculis principatum unius, hoc est monarchiam
According to St. Robert,
St. Robert's De Romano Pontifice (On the Roman Pontiff) treats the question "Should the Ecclesiastical Government be a Monarchy?" in book 1, chapters 5 through 9:
adapted from this answer to the question "What is the scriptural basis for a monarchical episcopate?"