Pope St. Pius X: The Church is a monarchy.

[Geremia]

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}

Also, 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").