Objection 1. It seems that God is not supremely "one." For "one" is so called from the privation of division. But privation cannot be greater or less. Therefore God is not more "one" than other things which are called "one."
Objection 2. Further, nothing seems to be more indivisible than what is actually and potentially indivisible; such as a point and unity. But a thing is said to be more "one" according as it is indivisible. Therefore God is not more "one" than unity is "one" and a point is "one."
Objection 3. Further, what is essentially good is supremely good. Therefore what is essentially "one" is supremely "one." But every being is essentially "one," as the Philosopher says (Metaph. iv). Therefore every being is supremely "one"; and therefore God is not "one" more than any other being is "one."
On the contrary, Bernard says (De Consid. v): "Among all things called one, the unity of the Divine Trinity holds the first place."
I answer that, Since "one" is an undivided being, if anything is supremely "one" it must be supremely being, and supremely undivided. Now both of these belong to God. For He is supremely being, inasmuch as His being is not determined by any nature to which it is adjoined; since He is being itself, subsistent, absolutely undetermined. But He is supremely undivided inasmuch as He is divided neither actually nor potentially, by any mode of division; since He is altogether simple, as was shown above (Question 3, Article 7). Hence it is manifest that God is "one" in the supreme degree.
Reply to Objection 1. Although privation considered in itself is not susceptive of more or less, still according as its opposite is subject to more or less, privation also can be considered itself in the light of more and less. Therefore according as a thing is more divided, or is divisible, either less or not at all, in the degree it is called more, or less, or supremely, "one."
Reply to Objection 2. A point and unity which is the principle of number, are not supremely being, inasmuch as they have being only in some subject. Hence neither of them can be supremely "one." For as a subject cannot be supremely "one," because of the difference within it of accident and subject, so neither can an accident.
Reply to Objection 3. Although every being is "one" by its substance, still every such substance is not equally the cause of unity; for the substance of some things is compound and of others simple.
Source: Summa Theologica (New Advent)