A: According to Ludwig Ott in Fundamentals of Catholic Dogma, the division of angels into nine orders is not a truth of faith but a free theological opinion.
At the turn of the sixth century Pseudo-Dionysius, drawing on references to angels in the Scriptures, divided the angels into three hierarchies with three choirs in each hierarchy. That became the common teaching of theologians and the Church.
According to Adolf Tanquerey in A Manual of Dogmatic Theology, St. Thomas puts the Seraphim, Cherubim and Thrones in the first hierarchy. In the second are the Dominations, Virtues and Powers. The third is composed of Principalities, Archangels and Angels.
Tanquerey, following St. Thomas, says the “Seraphim excel in the supreme excellence of all, in being united to God through charity.”